Infectious Diseases and Pathogens

Infectious Diseases

• An infectious disease is a host-centered concept, where living organisms cause harm and illness
• The human body serves as a habitat for many organisms, some of which can invade and cause harm
• Associations that harm or bother us, are considered infectious diseases are caused by pathogens
• Zoonosis refers to infectious diseases transmissible from animals to humans, where the animal is sick

Types of Pathogens

• Worms: Multicellular, parasitic organisms found in guts, tissues, muscles, etc
• Protozoa: Unicellular, parasitic microorganisms larger than bacteria
• Bacteria: Unicellular organisms, most of which are not parasitic; they can be aerobic, anaerobic, or tolerant to either condition, some form spores
  • Many species reside in our stomach and help with metabolism and digestion; much smaller than protozoa, can cause illness in various organs
• Viruses: Parasitic strands of DNA or RNA, very small in size
  • Lack a complete cell structure, possessing only genetic material and cannot survive outside a host for long, leads to serious illnesses

Body's Defense and Immunity

• Immune System: Distinguishes "self" from "foreign"
• Active Immunity: Develops upon first exposure to an antigen, prompting the body to produce antibodies
• Vaccination: Utilizes antigen preparation to stimulate active immunity, where weak pathogens are received, and the body produces its own antibodies
• Passive Immunity: Involves direct reception of antibodies, providing immediate protection but requires time to become active after vaccination
  • In COVID-19 vaccines, antigens need two to three weeks to produce the antibody and build up immunity
• Vaccinations are generally not 100% effective
• Herd Immunity: Offers practical protection where, if enough members of a group are immune, it is hard to maintain the chain of infection, limiting disease transmission

Strategies for Managing Transmission

• Segregation: Isolate sick individuals and quarantine those exposed to infectious agents
  • Quarantine involves separating all contacts of infected individuals
• Sanitation: Beneficial in reducing transmission but doesn't solve the problem entirely
  • Illness transmission is reduced through handwashing and using hand sanitizers
• Antibiotics: Treat illnesses caused by bacteria, but are ineffective against viruses
  • Over time, pathogen populations develop resistance due to their high reproductive rate and mutations
  • Overusing antibiotics, including as food additives, contributes to antibiotic resistance like MRSA
  • "Superbugs," including bacteria and fungi, resist multiple antibiotics
• Pesticides: Dangerous but effective tools for controlling vectors that transmit diseases like malaria and yellow fever

Modes of Transmission

• Closeness/Contact: Transmission occurs through
  • Droplet transmission (coughing, sneezing, talking, breathing) for diseases like diphtheria, tuberculosis, pertussis, influenza, COVID-19
  • Direct oral contact (kissing, touching) for strep, herpes simplex-1, infectious mononucleosis
  • Fomites, objects transmitting disease (towels, clothing)
• Airborne: Transmission in aerosols, where air circulation spreads diseases like fecal-oral diarrheal diseases
• Fecal-Oral: One person's diarrheal disease becomes the next person's infection because of fecal origin
  • Poor sewage control leads to waterborne transmission; chlorine helps by killing microorganisms in water
  • Occurs through soil and hand-to-mouth contact, observed in cholera, typhoid fever, dysentery, giardiasis, hepatitis A, Norwalk virus, and polio
• Other Means:
  • Non-fecal organisms transmit in water or soil (guinea worm disease, tetanus)
  • Foodborne transmission by vectors like houseflies

Global Infectious Disease Patterns

• Fast Global Spread: Traveling increases spread
• Total deaths caused by infections ~12.3 million
  • Respiratory infections (29%), diarrheal diseases (20%), and HIV/AIDS (14%) are prominent
• Worldwide deaths: 22% are due to infectious diseases, with highest rates in Africa (53%), Southeast Asia (27%), and the Eastern Mediterranean (25%)

Infectious Diseases and Cancer

• Cancer Risk: Infection can increase it
  • Chronic irritation leads to cell proliferation
• Significant cause of cancer: 18% can be attributed to infectious causes
  • Liver cancer (hepatitis B and C viruses, liver fluke), cervical cancer (human papillomavirus), and stomach cancer (Helicobacter pylori bacterium) are closely linked
• Income Level: Higher percentage of cancers being caused by infections is statistically proven in lower-income countries

Bacterial Pathogens

• Tuberculosis: Lung disease and one of the world’s deadliest, infecting 1/3 of the global population, annually affects approximately 9 million people, resulting in 1.4 million deaths
  • HIV-infected are highly susceptible due to compromised immune systems
  • Symptoms: Chest pain, bloody cough, fever, chills, night sweats
  • Transmitted: Droplets via cough, sneeze, kiss, spit
  • Preventative Measures: Vaccination and antibiotics
  • Most people contacted are exposed to the bacteria but do not get sick, common among travelers
  • TB cases in Canada are primarily imported
• Anthrax: A rare disease caused by Bacillus anthracis spore-forming bacteria
  • Bacteria form spores that enable long-term survival under harsh conditions, spreading during sporulation
  • Spores itself are harmless but become toxic when they germinate
  • Anthrax: All forms of anthrax can progress to septicemia and death, potentially used in bioterrorism attacks
    • There's no smell or taste, and the spores are too small

Clinical Forms of Anthrax

• Cutaneous Anthrax: Most common, occurs via skin contact with contaminated animal products with incubation period ranging from 1-12 days
  • Symptoms: Small bump progresses to painless ulcer; potential fever, headache, and swollen lymph glands but 20% of untreated cases are fatal
• Inhalation Anthrax: Inhalation of spores and most lethal form of the disease, scientists do not know how much spores have to be inhaled to get sick
  • Incubation period may vary from 1-60 days and begins as a viral respiratory illness
  • Respiratory failure is possible, along with meningitis
  • 75% of cases result in death
• Gastrointestinal Anthrax: Consumption of raw or undercooked contaminated meat with incubation period of 1-7 days
  • Nausea, loss of appetite, vomiting, fever, abdominal pain, bloody diarrhea with 25–60% mortality

Terrorist Anthrax

• Military and designated government personnel are usually vaccinated
• Mixed in powder for dispersion, watch out for
  • Suspicious Mail: Look for smells, powder spilling, unexplained oil stains
  • Environmental testing, diagnosis, cure (antibiotics and vaccine
• Plague: Caused by Yersinia pestis and can be used as a bioweapon

Forms of Plague

• Bubonic Plague: Contracted from infected fleas that bit infected rodents by infected fleas (vector) that bit infected rodents with incubation period of 2-6 days, bacteria travels through break in skin
  • Swollen lymph glands, fever, headache, chills, weakness but does spread from person to person via fleas
• Pneumonic Plague: Infection of lungs that is can be transmitted person to person but from animals to humans
• Septicemic Plague: Multiplying plague bacteria in blood with like symptoms, but no buboes

Viral Diseases

• Yellow Fever:
  • Causative Agent: Virus transmitted to humans by mosquitoes (Aedes aegypti)
  • Incubation: 3-6 days
• Symptoms:
  • First phase: Flu-like symptoms (fever, muscle pain, headache, vomiting)
  • Recovery: 85% recover after the first phase
  • Toxic phase (15%): Jaundice, internal bleeding, kidney failure
• Outcomes:
  • Victims: 50% of victims who develop the toxic phase recover
  • Fatal: Untreated victims die after 10-14 days
• Origins and Spread:
  • Origin: West Africa (Cameroon to Mauritania)
  • Climate Impact: Tropical and subtropical zones are more vulnerable than temperate zones
  • Transmission: Spread to the Americas during European Exploration
• Epidemics:
  • Philadelphia, USA (1793): Killed 4,044 people
  • Haiti (1801): killed 90% of Napoleon’s force
• Prevention: Failure to control mosquitoes allowed the spread, prompting quarantine efforts and sanitation improvements

Human Immunodeficiency Virus (HIV)

• Transmission: via blood, semen, vaginal fluid, or breast milk from infected mothers
• Impact: Causes a slow immune system failure leading to various illnesses
• Time of Original Discovery: December 1, 1981
• Current Status: considered to be a destructive pandemic in humans
• World numbers: 2018: 37.9 million people and caused 770,000 deaths
• Characteristics:
  • Duration from infection to death 11 years without treatment
  • Medications offer prolonged life expectancy
  • Antiretroviral drugs: Helps people with HIV to live longer, but expensive and inaccessible in some countries

Bird Flu and West Nile (Viral) Virus

• Bird Flu (Avian Influenza): Virus that causes illness and death in birds
  • Cannot eat chicken or eggs if infected
  • Transmission:
    • Human infections: Direct contact with infected bird (via bodily fluids)
    • High temperatures associated with proper cooking practices kill virus
• West Nile Virus (WNV): Mainly infects birds then mosquitos
  • Transmission: Bite of infected mosquito transmits to humans
  • Most infections: Asymptomatic or mild flu-like symptoms
  • Serious infection: Can lead to neuroinvasive West Nile Meningitis or Encephalitis
  • Prevention: Mosquito control measures and preventative wear via long sleeves and pants when outdoors due to being more common in the months of July and August

Protozoan Disease

• Malaria: Deadly protozoan disease carried by mosquitoes
  • Causative Agent: Plasmodium Protozoa
  • Transmission: Mosquitoes transmit it, where it reproduces in the human liver and bloodstream
• Disease Cycle: Cycle of infection between humans and their reproduction
• Burden: Kills millions annually
• Geography: Endemic in parts of South America, Africa, Asia, but no real cure or good treatment
• Treatment: Bed net usage with medications
• Success: Insecticide and antimalarial drugs helped eradicate the disease from Louisiana by 1951