An epidemic refers to a sudden outbreak of a disease that affects a large number of individuals within a specific region, community, or population. The number of cases in an epidemic exceeds the expected norm.
Epidemics of infectious diseases can occur when a virus, bacterium, or other pathogen becomes more potent, enters a new area where it was previously absent, or finds novel ways to infect individuals. It can also arise if people become more vulnerable to the disease or experience increased exposure.
Causes of Epidemics:
- Antigenic Variation: The surface proteins (antigens) of a virus help the immune system recognize and attack it. Changes in these antigens enable the virus to evade immunity and spread within previously immune populations.
- Drug Resistance: Some bacteria develop resistance to antibiotics through genetic mutations. Overuse of antibiotics can accelerate this resistance, making diseases like tuberculosis harder to treat, particularly with the rise of multi-drug-resistant TB (MDR-TB).
- Human Behavior: Lifestyle and behavioral changes can influence epidemic severity. A classic example is the 1854 Broad Street cholera outbreak, where removing a contaminated water source helped control the disease.
- Seasonal Factors: Certain diseases thrive in specific environmental conditions, such as changes in humidity and temperature. For example, cholera outbreaks in tropical regions are linked to wet seasons, while winter increases the spread of respiratory infections due to indoor crowding and weakened immune responses.
Types of Epidemics:
- Common Source Outbreak:
○ Occurs when multiple people fall sick due to exposure to a common contaminated source (e.g., food, water, or toxins).
○ Point-Source Outbreak: Happens when exposure is limited to a short period, like a group of people getting sick after eating at the same restaurant.
○ Continuous Common-Source Outbreak: Exposure persists over time, such as the 1854 cholera epidemic in London, caused by contaminated water from the Broad Street pump.
- Propagated Outbreak:
○ Spreads from person to person through direct contact.
○ Infected individuals can act as reservoirs, continuing the spread.
○ Example: The 2014 Ebola epidemic in West Africa, where the virus was transmitted via contact with infected bodily fluids, leading to widespread fatalities.
