Epidemiology

Morbidity vs Mortality:
- Morbidity rate: Number of people who get sick from a disease.
- Mortality rate: Number of deaths resulting from a disease.
Incidence vs Prevalence:
- Incidence: Number of new cases over a given time per 100,000 people, indicating disease spread risk.
- Prevalence: Total number of existing cases (new and old) indicating duration of illness.

Endemic: Diseases consistently present in a population (e.g., cold, flu, COVID-19).
Epidemic: An unusually high number of cases in a localized area.
Pandemic: A global spread of a disease.

Graphical representation helps identify epidemics:
- Normal cases appear flat; peaks indicate epidemic outbreaks.
To determine epidemic, monitor illness rates and plot over time.
Epidemic curves differentiate between localized (epidemic) and widespread (pandemic).

Pathogen reservoirs can vary in control measures:
- Wild animals: Difficult but manageable through control (vaccinations, animal control).
- Pets: Vaccinations can minimize disease spread.
- Humans: Symptomatic individuals show signs of disease; asymptomatic carriers do not exhibit symptoms but can still spread the disease (e.g., Neisseria gonorrhoeae).

Instances of gonorrhea peaked in the 1970s with a reported million cases annually; reduced to approximately 342,000 cases thereafter.
Factors affecting transmission rates include social behavior changes and the introduction of contraceptive methods leading to increased sexual activity and thus STDs’ transmission.

Recent increases in STDs (2019) such as:
- Chlamydia: 1.7 million cases (3% increase).
- Gonorrhea: Close to 600,000 cases, nearly doubled.
- Syphilis: Rising cases, including congenital syphilis (transmission from mother to child).

Contamination sources include:
- Botulism: Caused by Clostridium botulinum (foodborne illness) typically due to improper canning practices.
- Tetanus: Caused by Clostridium tetani found in dirt affecting wounds, preventable through vaccination.

Direct contact: Physical interaction with infected individuals or parasites.
Indirect contact: Transmission via fomites—objects that carry pathogens.
Droplet transmission: Coughing or sneezing into the air.
Food and water: Contamination sources.
- Cryptosporidium parvum: Resistant to chlorine treatment, presents a major issue in waterborne illnesses requiring boiling for safe use.

Epidemiological studies track demographics of infections by age, occupation, behaviors.
Example: 19th-century chimney sweeps and association with testicular cancer due to soot exposure.
- Air Force personnel exposed to aviation fluids also show increased cancer rates.

John Snow's mapping of cholera cases revealed contaminated water source leading to public health interventions.

Common pathogens:
- E. coli, Staphylococcus aureus, Pseudomonas aeruginosa (resistant organism).
Major infection risk factors include compromised immune systems, surgical equipment contamination.

The innate immune system is the body's first line of defense comprising:
- Skin: Waterproof barrier, dead cells, sweat,
- Lysosomes: Breaks down bacterial cell walls.
- Tightly packed epithelial cells prevent pathogen entry.

Neutrophils: Most abundant, short-lived phagocytes active in infection response.
Macrophages: Long-lived, tissue-residing phagocytes with strong infection-fighting capabilities.
Dendritic Cells: Sample foreign antigens to alert the body.
T Cells & B Cells: Engaged in adaptive immunity which responds to specific pathogens but not covered in this section.

Cytokines: Signaling molecules for immune communication including:
- Chemokines: Direct immune cells to infection sites.
- Interferons: Antiviral signaling molecules.
- Interleukins: Involved in inflammatory responses.

Complement Cascade:
- Activated by antibodies hitting antigens, foreign proteins, or even random interactions.
- Functions include:
- Inflammation: Facilitates blood flow to infections.
- Membrane Attack Complex (MAC): Produces holes in pathogen membranes.
- Opsonization: Marks pathogens for destruction by phagocytes.
Activation Methods:
- Encountering antigen-antibody complexes, specific carbohydrate binding, or indiscriminate interactions.

Diapedesis: Movement of neutrophils from blood into tissues during inflammation.

Fever: Body temperature exceeding 37.8°C (100°F) mediated by pyrogens improving immune response and inhibiting pathogen growth.
- Must balance high enough temperature to affect pathogens without harming human proteins.

The upcoming discussion will focus on the adaptive immune system which varies greatly across individuals based on previous exposure and environmental factors.