Diseases: Cause and Control PDF

# Diseases: Cause and Control ## In the Chapter - A brief introduction to communicable, non-communicable, endemic, pandemic and sporadic diseases. - Modes of transmission. - Meaning of each of the above with examples. - Bacterial, viral, protozoan, helminthic diseases. - Bacterial: *cholera*, *t...

# Diseases: Cause and Control ## In the Chapter - A brief introduction to communicable, non-communicable, endemic, pandemic and sporadic diseases. - Modes of transmission. - Meaning of each of the above with examples. - Bacterial, viral, protozoan, helminthic diseases. - Bacterial: *cholera*, *typhoid*, *tuberculosis*. - Viral: *AIDS*, *chicken pox*, *hepatitis*. - Protozoan: *Malaria*, *amoebic dysentery*, *sleeping sickness*. - Helminthic: *Ascariasis*, *taeniasis*, *filariasis*. - Symptoms and measures to control the above diseases. - Scientific names of causative agents not required. ## 16.1 What is a Disease? The term "disease" is a condition in which the normal functioning of the body is disturbed. Many times such conditions are not very serious and get cured, but sometimes they become serious and even fatal. Disease is a departure from normal health through structural or functional disorder of the body. Early man looked at disease as an act of God or evil spirits. He used magical methods and sorcery for the treatment of the disease. Today, science looks at every disease to be a condition which has a definite cause and thereby making it possible to prevent it or cure it. To make people conscious of being healthy and disease-free, we celebrate: **World Health Day - April 7** ## 16.2 Categories of Diseases Diseases can be categorized in several ways, for example based on: 1. **Extent of occurrence:** - Endemic - Epidemic - Pandemic - Sporadic. 2. **Communicability:** Whether infectious (caused due to germs) or non-infectious (due to the body's own poor functioning). 3. **Kinds of pathogens:** Whether bacterial, viral, protozoan or some other types of germs. 4. **Kinds of transmitting agents:** Whether air-borne, water-borne, food-borne, or insect-borne, etc. ## 16.3 Categories of Diseases Based on the Extent of Occurrence 1. **Endemic diseases:** When the disease is found in a certain area only attacking a fewer number of people, e.g. *yellow fever* in certain African countries, *goitre* in sub-Himalayan regions. 2. **Epidemic diseases:** When the disease breaks out and spreads from place to place affecting large numbers of people at the same time, e.g. *plague in India in 1994* (from Surat to many other places). 3. **Pandemic diseases:** When the disease is widely distributed worldwide, e.g. *AIDS*, *COVID-19*. 4. **Sporadic diseases:** When there are scattered individual cases of a disease, e.g. *malaria* and *cholera*. ## 16.4 Categories of Diseases Based on Communicability Diseases can also be classified into two major categories: 1. **Non-communicable or Non-infectious diseases:** In these diseases, there is no germ of any kind and they cannot spread from a patient to another person by contact or by any other method, e.g. *diabetes*, *Arthritis*, *mental illness*, *cancer*. 2. **Communicable or Infectious diseases:** These diseases are caused by germs. The germs somehow reach from an infected person to a healthy person, thereby spreading the disease to the healthy person as well. These disease-causing germs are known as *pathogens* and their transmission from one person to another is called *infection*. An infectious disease usually does not appear immediately after the infection, but it may take some time varying from a few hours to a few days; this period is called the *incubation period*. Common examples: *cholera*, *smallpox*, *malaria*, *chicken pox*. ### Progress Check | Column I | Column II | |---|---| | Plague | Pandemic | | Malaria | Endemic | | Goitre | Endemic | | AIDS | Pandemic | ## Classify the following diseases into communicable and non-communicable diseases: - *Cholera* - *Beri-beri* - *Color blindness* - *Diabetes* - *Malaria* - *Plague* - *Heart attack* **Communicable Diseases:** - Cholera - Malaria - Plague **Non-communicable Diseases:** - Beri-beri - Color blindness - Diabetes - Heart attack ## 16.4.1 Non-Infectious Diseases (Extra information - Not in syllabus) Given below is a simplified classification of non-infectious diseases with some common examples. 1. **Nutritional deficiency diseases:** *Beri-beri*, *scurvy*, *goitre*, *kwashiorkor*, *night blindness*. 2. **Metabolic diseases:** *Diabetes* (*diabetes mellitus*), *goitre* (*hypothyroidism*). 3. **Genetic diseases:** *Haemophilia*, *Thalassemia*. 4. **Allergies:** *Hay fever*, *asthma*. 5. **Degenerative (ageing) diseases:** *Arthritis*, *cataract*. **Arthritis** is the inflammation of joints due to deteriorating metabolism. Arthritis is caused even due to infections. **Mental illness:** *Depression*, *schizophrenia*. **Cancer:** *Breast cancer*, *prostate cancer*, *leukemia*. Cancer is an abnormal multiplication of cells. Their uncontrolled divisions may lead to a tumor. Cancer occurs most commonly in those tissues in which cell division is a normal activity, e.g. skin, liver, lining of stomach, uterus, breasts, etc. Cancers are often fatal. Early detection and treatment improve chances of survival. Many causes are suggested for cancer which include chemicals, tars from tobacco smoking, tobacco chewing, drugs, pollution, certain radiations, and even certain viruses. Any agent that causes cancer is called a carcinogen. **Why the name - Cancer?** Cancer has been given this name because it adheres to any part of the body which it siezes upon in an obstinate manner like the well known crab Cancer. ## 16.4.2 Communicable or Infectious Diseases The infectious diseases are always caused due to a disease-causing organism called a *pathogen* (*pathos*: disease, *gen*: producing). The pathogens include a wide variety of organisms. Following is a list of some common pathogens and the diseases they cause. | Pathogens | Diseases | |---|---| | Bacteria | *Cholera*, *typhoid*, *tuberculosis*. | | Viruses | *AIDS* (*HIV*), *chicken pox*, *hepatitis*. | | Protozoa | *Malaria*, *amoebic dysentery*, *sleeping sickness*. | | Helminths (worms) | *Ascariasis*, *taeniasis*, *filariasis*. | ### Incubation period Incubation period is the period between the entry of germs and the appearance of the first symptoms of the disease. | Disease | Incubation Period | |---|---| | Pneumonia | 1-3 days | | Diphtheria | 2-5 days | | Cholera | 2-6 days | | Tetanus | 4-20 days | | Gonorrhea | 5-10 days | | Poliomyelitis | 7-14 days | | Typhoid | 7-14 days | | Measles | 10-12 days | | Smallpox | 10-15 days | | Whooping cough | 12-26 days | | Mumps | 14-21 days | | Chicken pox | about 1 month | | Rabies | about 1 month | | Leprosy | up to several years | | HIV/AIDS | up to 12 or more years | All infectious diseases have a certain incubation period. Incubation periods (from the shortest to the longest) of some diseases are given in Table 16.1. ## 16.5 Diseases Caused by Bacteria Types of Bacteria: You have already read about types of bacteria in this book. Some of the diseases caused by bacteria in humans are as follows: 1. **Cholera:** The disease attacks the intestinal tract, and is caused by a special bacterium called *vibrio* (*Vibrio cholerae*). Incubation period is a few hours to 6 days. The patient suffers from vomiting and loose motions. There is very little or no urination because of the shortage of water in the body. Urea accumulates in the body, which is poisonous. The patient may die if not treated properly and promptly. The treatment includes saline water injection to supply water to the blood. The cholera germ is spread through food and water (faeco-oral route). The contamination results due to dust and by direct transport through flies. The flies sit on excreta (faeces) of the patient and mechanically carry the germs which stick to their spiny legs and body, to the exposed food that is eaten up (through the mouth, i.e. oral). **Prevention methods include:** - Good sanitation. - Killing flies. - Keeping the food and cut fruit properly covered to prevent the flies reaching them. - Boiling water for drinking. - Eating well-cooked food. 2. **Typhoid Fever:** Its major symptom is a continuous fever which usually rises in the afternoon. Reddish eruptions appear on the chest and abdomen. The causative germ (*Salmonella typhi*) is a bacterium which attacks the intestines. The patient passes out the germs in his excreta. Flies and direct contamination of food, and particularly of milk, spread the disease. The incubation period is about 7-14 days, although it may be as long as 30 days. After recovery, in about 2-3 weeks, the patient is left very weak. Relapse is not uncommon. Some people after recovery are carriers of the disease; they may still have the germs. *Chloromycetin* is a very effective antibiotic in treating the disease. 3. **Tuberculosis:** It inhibits the growth of bacteria. **Prevention:** BCG vaccination has been found useful in developing immunity for tuberculosis. **March 24 is observed as anti-tuberculosis day.** | Disease | Incubation period | Mode of transmission | Symptoms | Prevention | |---|---|---|---|---| | Tuberculosis | 2-10 weeks | Through air, dust, sputum of infected person | Persistent cough, afternoon fever, bloody mucus, loss of weight and fatigue, chest pain, breathlessness | Streptomycin antibiotic, BCG vaccination and isolation of patient. | | Cholera | Few hours to 6 days | Contaminated water, food and drinks, spread by flies | Acute diarrhoea, vomiting, abdominal pain, dehydration, reduced urination | Proper sanitation, control of flies, avoid exposed food, anti-cholera injection | | Typhoid | 7-14 days | Contaminated water, milk, through flies | High fever rise in afternoon, acute head ache, red nose, reddish eruption on the chest, diarrhoea | Proper sanitation, Chloromycetin antibiotic and anti-typhoid vaccination | ***Extra - Not in syllabus but useful to know:*** **Tetanus:** This is also popularly called lockjaw, because in its last phase the patient's jaws are tightly closed or locked. Death results in most cases. It is caused by the tetanus bacterium (*Clostridium tetani*). The germ enters the body through deep wounds. Any cut in the skin due to a fall on the ground can be a source of tetanus infection. The incubation period is from 4-20 days. The germ is frequently present in horse- and cow-dung. **Prevention:** Tetanus can be prevented by previous vaccination or by giving anti-toxin after getting serious wounds. **Syphilis:** It is one of the venereal diseases now better called *sexually transmitted diseases (STD)*. Its bacterium (*Treponema*) grows in genital tracts and causes a pus-like discharge. Syphilis is very dangerous and may extend to all other systems of the body, sometimes leading to death. Contact with lips also spreads syphilis. The incubation period is 1-12 weeks. Antibiotics, and particularly penicillin, are very good in curing the bacterial diseases. **Diphtheria:** This dangerous disease strikes young children. The germ (*Corynebacterium diphtheriae*) is spread by discharge from the throat of infected persons by sneezing, coughing, etc. The incubation period is 2-5 days. It affects the throat and grows very rapidly, often taking the life of the patient. It is prevented by inoculating vaccine. **Whooping cough (Pertussis):** It primarily occurs in children. The germ is a bacterium (*Haemophilus pertussis*). It is spread by discharges from the throat of the infected person. Incubation period is 10-15 days. It can be prevented by vaccination. **Triple vaccine (also called DPT):** is a combined vaccine given by injections to young babies to protect them from the three diseases: *diphtheria*, *whooping cough (pertussis)*, and *tetanus*. **Pneumonia:** It is caused by a bacteria (*Streptococcus pneumoniae*) by contact or by air. It is a serious disease of the lungs. Incubation period is 1-3 days. Antibiotics like penicillin are very effective in curing it. ## 16.6 Diseases Caused by Protozoa 1. **Malaria:** Malaria is caused by a protozoan *Plasmodium* and is most commonly spread to humans through the bite of an infected female *Anopheles mosquito*. When an uninfected female mosquito bites an infected person, the parasite is sucked in along with the person's blood. Once inside the mosquito's body, the parasite grows and multiplies within its gut. This mosquito is now infected and is known as a "malaria vector". When this vector mosquito bites a healthy person, it transmits the infective stage of the parasite along with its saliva into the bloodstream of the person. The parasite then grows and multiplies in the liver cells and goes on to infect and destroy the red blood cells of the person. The incubation period is about three weeks. Chill and high fever are repeated on the third or fourth day. Various drugs such as *quinine*, *paludrin*, *camoquin*, etc., are useful in treatment. The disease is common in hot and humid countries. **Control measures include:** - Destruction of the mosquito at all stages and to avoid getting a mosquito bite by using mosquito nets or repellants. 2. **Amoebic Dysentery (Amoebiasis):** It is caused by a kind of amoeba called *Entamoeba histolytica*. It causes destruction of the lining of the large intestine and diarrhoea with griping pain and discharge of mucus and sometimes blood in the stools. Infection is by contamination of food, especially by flies. The incubation period is about one week. Proper sanitation and protecting food from dust and flies prevents spreading of the disease. 3. **Sleeping Sickness:** It is a disease caused by the flagellated protozoan *Trypanosoma bruce gambiense* and *Trypanosoma brucei rhodesiense* gets transmitted through the bite of the Tsetse fly. Its symptoms are fever, headache, itchiness and joint pains in the first phase. The patient later on suffers from confusion, poor coordination, numbness and trouble in sleeping (hence referred to as sleeping sickness). This phase is also known as the neurological phase and can be fatal at this stage. **Treatment:** Administering drugs like *melarsoprol*, etc. ### Extra - Not in syllabus but useful to know: | Disease | Incubation period | Mode of transmission | Symptoms | Prevention | |---|---|---|---|---| | Tetanus (lock jaw) | 4-20 days | Through cuts of wounds in the skin, enters through blood into spinal cord | Locking of the jaw due to muscle spasm | Avoid wound contact with infected persons. | | Syphilis | 1-12 weeks | Sexually transmitted, close contact | Ulcer on penis or rectum, lips, tongue nipple; skin rash and fever | DPT vaccine, no sexual contact with infected person | | Diphtheria | 2-10 days | Droplet infection while coughing and sneezing, contact | Sore throat, skin ulceration, fever, high fever, rash on throat | DPT vaccine, maintain sanitation | | Whooping Cough | 10-15 days | Contact, droplet infection of throat | Cold with running nose, bouts of coughing, whooping develops as a sudden bout of noisy breath at the end of cough, vomiting | DPT immunization | | Pneumonia | 1-3 days | Contact or by air | Inflammation of lungs., high fever, breathing problem and fatigue | Avoid malnutrition and contact | | Leprosy (Mycobacterium leprae) | Several years | Contact, highly contagious | Nervous loss of sensation, paralysis and deformity | Vaccine, good nutrition, sanitation | | Gonorrhea (Neisseria gonorrhoeae) | 3-10 days | Sexual contact | Pain in passing urine, discharge of pus, pain | Avoid sexual contact with infected person | ## 16.7 Diseases Caused by Parasitic Worms There are numerous worms which cause diseases in man; some more important ones are as follows: 1. **Ascariasis:** A disease caused by common round worm (Fig. 16.2). Round worm is an elongated cylindrical worm about the size of an earthworm, females being slightly longer. The male can be recognized by a slightly curved hind end. These worms may be found in the intestines where they absorb digested food from the host. The female lays several thousand eggs per day, which pass out along with the patient's faeces. The eggs get scattered in the soil, reaching even vegetables in the fields. Children playing on the ground may get the microscopic eggs on their hands and the infection can occur by eating with unwashed hands or through unwashed raw vegetables. 2. **Taeniasis:** A disease caused by tapeworms called *Taenia solium*. There are two common tapeworms. One is spread by eating infected pork and the other by infected beef. The tapeworm remains in the intestine where it may reach a length of a metre or so (Fig. 16.3). It absorbs most of the host's digested food and the patient becomes terribly weak. The mature worm breaks off a small segment from its tail end which passes out with the faeces. These segments contain eggs. When the eggs are accidentally ingested by a pig or cow they hatch and form larval stages which settle in the muscles of their new host. When man (the final host) eats raw or imperfectly cooked pork or beef the worm matures in his intestine. 3. **Filariasis:** It is a tropical disease also known as 'Elephantiasis'. It is caused by the filarial worm *Wuchereria bancrofti*. It is transmitted through the bite of the Culex mosquito. These tiny worms lie in the lymphatic system and connective tissues of the human body mainly the limbs. Legs become swollen resembling those of the elephant, hence the name 'Elephantiasis'. The swelling is due to the blockage of lymph circulation by the worms resulting in inflammation of lymph glands and lymph vessels. As symptoms are enlargement of limbs/ ankle, fever with chills in acute cases. **Treatment:** Eradication of the vector and use of antibiotics and anti-inflammatory analgesics. ## 16.8 Viral Diseases ### 16.8.1 What are Viruses? The viruses are extremely small substances made of nucleic acids (RNA) and proteins. They are visible only through an electron microscope. - They do not live freely in nature. They can live only inside other cells. - They take over the metabolism of their host cell in their own favour producing more viruses and usually killing the host cells. - The viruses can be cultured on living tissue in the laboratory and can be crystallized and stored on the shelf. - The viruses are highly specific. They attack only one kind of host and only certain tissues. For example, the virus of *myxomatosis* which is fatal to rabbits has no effect on man and similarly the rabbits do not catch the common cold and flu. But there are some exceptions. The rabies virus normally affects the dog and its relatives, but it can even cause the disease in humans frequently leading to death. - Virus is like a gene with no cell of its own. It dictates the host cell to produce more of its own kind of virus particles only. The host cell ultimately dies and the liberated virus particles attack new host cells. - Viruses are considered as a connecting link between living beings and non-living beings. | Features which support them as living beings | Features which support them as non-living beings | |---|---| | 1. They can multiply inside a host cell which can be considered as reproduction. | 1. They cannot respire, metabolize, grow or multiply by themselves outside a host cell. | | 2. They command the host cells and behave like living beings inside the host (plants/animals/ human beings). | 2. They can be crystallized and made inactive for indefinite length of time. | ## 16.8.2 Common Viral Diseases Some common viral diseases are the *poliomyelitis*, *common cold*, *influenza*, *AIDS*, *chicken pox*, *smallpox*, *mumps*, *rabies*, and *hepatitis*. **World Rabies Day - 8th September** Louis Pasteur (1822-1895) established that all infections are caused by germs. He was the first to have successfully treated a boy badly bitten by a mad dog by giving him injections of weakened germs up to the 14th day. Most immunizations today are based on Pasteur's principles. ## In Case of Animal Bites **Do's** - Wash wound with soap and water. - Apply available disinfectant (Povidone iodine Spirit/Household antiseptic). - Contact doctor for timely & appropriate treatment with anti-rabies vaccines and immunoglobulins. **Don'ts** - Don't touch the wound with bare hands. - Don't apply irritants like soil, chillies, oil, herbs, chalk, betel leaves. ## 1. HIV/AIDS. (Human Immunodeficiency Virus/ Acquired Immuno Deficiency syndrome) AIDS was first recognised in the USA in 1981. Since then an increasing number of cases are reported every year from practically all over the world. In India alone, it is estimated that there are more than 2,00,000 confirmed cases of AIDS patients. The disease is spreading at an alarming rate, and it has no cure as yet and no vaccine so far. The disease is almost fatal. People in the age group 20-39 are more susceptible to getting AIDS. ### AIDS Is the Last Stage of HIV Infection The causative germ of AIDS is a virus named *HIV* (*Human Immunodeficiency Virus*) shown in Fig. 16.4. It has been detected in body fluids like blood, semen, saliva, tears and urine. It attacks the immune system (i.e. the cells that fight against infections) and the patient suffers seriously from even minor infections and the appearance of other diseases. Even cancers appear when the immune system fails. The incubation period i.e. the time between receiving the infection and the appearance of symptoms may be more than 10-12 years. During this period, the infected persons show positive results for HIV and they are popularly called *HIV-positive*. Most individuals, when AIDS is fully developed, show various symptoms such as severe weight loss, fever, night sweats, and infective illnesses. **It is caused through:** - Initial contact - Contaminated blood transfusion - Mother to child transmission during birth - Shared unsterilised injection needles **Other infections may include cancers, nervous system infections, infections involving eyes, etc. during this period.** ### Spreading of AIDS **AIDS is NOT transmitted by:** - Contact with patients clothes and other articles - Shaking hands - Eating together - Sharing bathrooms and toilets **HIV infection may be introduced through:** - Injection needles if shared by more than one person may introduce the virus from one individual to another. The disease is quite common in drug abusers. For the same reason, doctors in hospitals now use only disposable syringes which are used just once. - **Sexual Intercourse:** It is more common in homosexual males. - **Contaminated Blood transfusions:** In many situations such as excessive bleeding resulting from injury, or during surgery, etc., the patients have to be give blood transfusions. Some children are born with the disease *thalassemia* with defective haemoglobin of the blood. Such children have to be given regular blood transfusions usually every 3-4 weeks and very often the blood transfused is from professional donors. Such children run the risk of getting AIDS. - **Mother to child transmission:** The germ from the infected mother may cross through the placenta and reach the embryo in the womb. **World AIDS Day - December 1** ## 2. Chicken Pox (Varicella zoster) Chicken Pox is a contagious disease caused by Herpes virus occurring mainly in children, but can also affect adults. The disease spreads quickly by close contact with an infected person. Its symptoms are highly irritating rashes that appear on the body starting near the chest and back and gradually spread to arms, legs, face and head. The rashes first appear as a pink spot and rapidly changes to a watery blister. The blister gradually shrivels up and soon dries forming scabs, this time is the infectious period. **Treatment:** Bed rest, keeping the rashes clean and dry, do not prick the blister, calamine lotion can be applied to reduce itching, use of neem leaves also helps. Vaccination of live attenuated vaccine containing Varicella is given to children of the age of 12 to 18 months for active immunization. ## 3. Hepatitis Hepatitis is active disease is caused by 5 strains of viruses namely Hepatitis A virus, Hepatitis B virus, Hepatitis C virus, Hepatitis D virus and Hepatitis E virus. The virus causes inflammation of the liver. It is caused by contaminated food, water, contaminated syringes and blood transfusions. - **Hepatitis A virus:** Has an incubation period of 14 to 45 days and is most common in children and young adults, mainly transmitted by contaminated food and water. - **Hepatitis B virus:** Has an incubation period of 6 to 26 weeks and individuals of any age can be affected, mainly transmitted through contaminated syringes and possibly equipment. It can produce cirrhosis and transfusion cancer of the liver. Symptoms of Hepatitis C and D are high temperature, headache, joint pains, loss of appetite with a general feeling of illness, nausea and vomiting. After 3 to 10 days, jaundice may develop with deep yellow urine and light coloured stools. **Treatment:** Bed rest until fever has settled. Take high calorie diet with limited or no protein and fat. Wash hands after handling patient's bed pan and clothes. ## 4. Coronavirus Disease 2019 (COVID-19) COVID-19 disease is caused by *Severe acute respiratory syndrome corona virus 2 (SARS-CoV-2)*, which was first identified in Wuhan, China in December 2019. The disease spread all across the globe and has killed more than 5 million people till date, making it one of the deadliest pandemics in the history. The SARS-CoV-2 virus has undergone several modifications in its genetic material, giving rise to a number of viral strains over the course of the pandemic such as Delta, Omicron, etc. **Transmission:** COVID-19 is an airborne disease spread via contaminated air containing microscopic viral particles or ‘virions.’ **The disease is mainly transmitted via the respiratory route. Healthy people are infected when they inhale droplets and small airborne particle exhaled by those who are already infected. The infection is more likely to spread when healthy persons are in close proximity with diseased persons. The incubation period of COVID-19 ranges from 1-14 days, most commonly being around 5 days.** **Symptoms:** Common symptoms of this disease are fever, cough, sore throat and breathing difficulties. In some cases, headaches, body ache and loss of smell and taste are also reported. In some cases (especially in old people and people with other health conditions like cardiovascular disease, diabetes, respiratory disease, cancer, etc.), symptoms are severe enough to cause hospitalization. Most cases recover within 1-2 weeks while some cases may take upto 6 weeks to recover. **Prevention:** Preventative measures advised by WHO to reduce the chances of infection are: - Staying at home - Regularly washing hands with soap and water - Avoiding crowded spaces - Wearing a mask in public and maintaining a safe distance - Getting vaccinated. Furthermore, people who have been either infected or exposed to the virus are advised to isolate and/or quarantine themselves for about 14 days to prevent the spread of the virus. **Treatment:** There is no specific cure for this disease till date. Good personal hygiene, healthy diet and proper intake of fluids aid in faster recovery. However, in severe cases, hospitalization and oxygen support is often needed. Vaccines are highly effective in preventing the viral infection. Mass vaccination programmes driven by governments across the world have been instrumental in preventing the disease and slowing the spread of the virus. Many countries, including India, have developed their own vaccines against COVID-19. ## How Do the Viruses Differ From Bacteria? | Viruses | Bacteria | |---|---| | 1. Very small (visible only by electron microscope) | Larger (can be seen by light microscope). | | 2. Acellular. | Single-celled. | | 3. Have no metabolism. | Have metabolism. | | 4. Take no food by any method. | Take food by absorption. | | 5. Do not grow and do not divide. | Grow in size and divide to produce more bacteria. | | 6. Can be crystallized. | Cannot be crystallized. | | 7. Command the host cell to produce virus. | Self reproduce. | | 8. All produce diseases in man, animals or plants. | Some harmless, some useful and some disease-causing. |