Principles of Disease and Epidemiology

Principles of Disease

• Pathology studies disease, focusing on disease etiology and pathogenicity.
• Etiology is the primary concern, addressing the cause of the disease.
• Pathogenicity, the secondary concern, studies how the disease develops.
• Infection occurs via the invasion/colonization of the body via pathogenic organisms.
• Disease refers to an abnormal bodily state, where the body cannot perform normal functions.

Normal Human Microflora

• Normal microbiota are the microbes that live on or in the human body.
• A typical human body consists of approximately 1 x 10^13 human cells.
• The human body harbors approximately 1 x 10^14 bacterial cells.
• Transient microbiota are microbes present for short periods before disappearing.
• Microflora is localized in exposed regions of the body, such as the skin, respiratory, intestinal, and urinary tracts.

Role of the Microflora

• The host benefits, because microflora prevents pathogen growth.
• Microflora produces harmful substances to protect against invading microbes via microbial antagonism.
• Microflora engages in competitive exclusion to prevent pathogen growth by consuming available nutrients.
• Clostridium difficile is inhibited in the large intestine by microflora; antibiotics can eliminate the microflora, allowing C. difficile infection, which can cause fatal colon inflammation.
• E. coli in the large intestine produces vitamin K and vitamin B as major source.

Opportunistic Pathogens

• Opportunistic pathogens are normal microflora that don't typically cause disease.
• Opportunistic pathogens can cause disease if transferred to another part of the body, the host becomes immunocompromised (e.g., AIDS), or the normal microflora is disturbed.
• E. coli resides in the large intestine, but can cause infection if transferred to the urinary tract.
• Streptococcus pneumoniae, found in the respiratory tract, can cause pneumonia if the host is weakened, (e.g., after a cold).

Etiology and Koch's Postulates

• Koch’s Postulates were developed under the germ theory of disease.
• They allow determination of disease-causing microorganisms.
• The same pathogen must be present in every case of a given disease.
• The pathogen must be isolated and grown in a pure culture.
• The pathogen from the pure culture should cause disease when inoculated into a healthy lab animal.
• The same microbe should be isolated again from the inoculated individual.

Exceptions to Koch's Postulates

• Some bacteria will not grow in pure culture, such as Treponema pallidum, the causative agent of syphilis.
• Some pathogens cannot be used to infect lab animals, such as HIV.
• Several different microorganisms can sometimes cause the same disease, like pneumonia.
• One pathogen can sometimes cause many different diseases, such as Streptococcus pyogenes, which causes strep throat, skin infections, and scarlet fever.

Clarification of Disease

• Symptoms are what the patient feels, (e.g., pain or malaise).
• Symptoms usually are subjective and variable.
• Signs are objective changes that a physician can measure, (e.g., lesions, swelling, fever, or paralysis).
• Syndrome refers to a specific group of signs and symptoms accompanying a particular disease.
• A communicable disease spreads from one host to another, (e.g., chickenpox, measles, or STDs).
• Contagious diseases spread easily, (e.g., chickenpox and measles).
• Non-communicable diseases do not spread between hosts, (e.g., salmonellosis).

Occurrence of Disease

• Sporadic diseases occur only occasionally, such as hamburger disease.
• Endemic diseases have low, constant levels, like malaria or the common cold.
• Epidemic diseases occur in high numbers over a short time, like influenza.
• Pandemic diseases are epidemics that occur worldwide, like influenza and bird flu.
• Emerging infectious diseases are newly identified with notable number of cases, like West Nile Virus or SARS.

Severity and Duration of Disease

• Acute diseases develop rapidly and last a short time, such as influenza.
• Chronic diseases develop slowly and last a long time, such as tuberculosis.
• Latent diseases are inactive for a time before reactivation, Cold sores: Herpes Simplex Virus-I

Extent of Disease

• Local infections are confined to a small area of the body.
• Systemic infections spread microbes or toxins throughout the body.
• Septicemia refers to a systemic infection of the blood.
• Bacteremia is the presence of bacteria in the blood.
• Toxemia is the presence of toxins in the blood, as with Clostridium tetani in a cut, releasing tetanus toxin.
• Viremia is the presence of virus particles in the blood.

Stages of Disease

• Incubation Period spans the time between infection and the first signs or symptoms.
• Prodromal Period involves early, mild symptoms such as malaise.
• Period of Illness involves the most severe signs and symptoms; an active immune response may cause some signs and symptoms, (e.g., fever), and death may occur if the disease isn’t overcome.
• Period of Decline is when signs and symptoms gradually subside; it can last hours or days, and the patient is susceptible to secondary infections.
• Period of Convalescence occurs when recovery takes place; the pathogen may still be present and spread, and a person can remain a carrier for months.

Spread of Infection

• Reservoirs are infectious agent sources, including objects, food, and living things.
• Human reservoirs are people who harbor and transmit pathogens.
• Germs transmit at any disease stage: incubation, prodromal, illness, or convalescence.
• Carriers are individuals transmit pathogens without showing signs of disease, such as Typhoid Mary who carried Salmonella typhi.

Animal and Non-living Reservoirs

• Zoonoses are animal diseases transmissible to humans (e.g., influenza from birds and pigs; rabies from bats, skunks, and foxes; Lyme disease from field mice via ticks).
• Soil and water are key non-living reservoirs.
• Clostridium tetani in soil can cause tetanus if bacteria enter the body through a cut.
• Fomites are non-living objects that spread pathogens, (e.g., used tissues or syringes).

Transmission of Disease

• There are three primary routes of transmission.
• Contact transmission involves direct contact, like person-to-person spread of chlamydia and other STDs.
• Contact transmission involves indirect contact via fomites.
• Contact transmission involves droplets that spread over short distances (less than one meter) via coughing, sneezing, or talking, which can spread influenza.
• Vehicle transmission involves pathogens transmitted by a medium.
• Vehicle transmission (waterborne) involves pathogens such as E. coli due to improperly treated sewage.
• Vehicle transmission (airborne) involves pathogens of less than one meter- dust particles, droplets, or Mycobacterium tuberculosis.
• Vehicle transmission (foodborne) involves food poisoning from Bacillus cereus or salmonellosis.
• Vector transmission involves animals that carry pathogens from one host to another.
• Vector transmission (insect): West Nile Virus gets transmitted via birds, mosquitoes, and humans.
• Vector transmission(insect): Lyme Disease gets transmitted Field Mice, Ticks, and Humans.

Nosocomial Infections

• Nosocomial infections are hospital-acquired, occurring in 5-15% of patients.
• Such infections stem from pathogens in hospitals, immunocompromised patients, and transmission chains.
• Transmission chains involve direct contact from staff or patients, indirect contact from fomites, and airborne spread through ventilation systems.

Controlling the Spread of Disease

• Proper hand washing is the MOST effective hygiene to combat the spread of disease.
• Proper sterilization, disinfection, and sanitation are also important.
• The proper preparation of food and water helps prevent the spread of the disease.
• Proper sanitation and sewage treatment also combats the spread of disease.
• Controlling insect vectors prevents the spread of disease, like mosquito control in West Nile spread.
• Vaccinating domestic animals controls the spread of disease, like against rabies.
• Avoiding contact with carriers, (e.g., using latex gloves), controls disease spread.
• Control disease spread through quarantine, by removing infected individuals from society, (e.g., tuberculosis sanitoriums).
• Isolating people highly susceptible to a particular disease controls its spread, (e.g., bone marrow transplant patients).