Epidemic :A clear excess in the number of cases of a given disease (over the expected level) i.e in comparison with the expected level or what is expected in a given time period .

Expected level can be known by comparing with previous reports , If expected number was zero and we have two cases , it means an epidemic . Iraq is free from polio then the appearance of two cases of polio means an epidemic .We usually compare with average of the previous five years .

Every epidemic should be reported but not every one should be investigated , a case of HIV or cholera should be reported and investigated , while 30 cases of gastroenteritis do not need investigation.

Endemic – Persistent, usual,or expected health-related state or event in a defined population over a given period of time

• Pandemic – Epidemic affecting a large number of people, many countries, continents, or regions

Endemic vs. Epidemic

Endemic Epidemic

No.

of C

ases o

f a D

isease

Time

Common source, propagated, and mixed epidemics 1-Common source

Point

2-Propagated

Common source Tend to result in more cases occurring more rapidly

and sooner than host-to-host epidemics

Identifying and removing exposure to the common source typically causes the epidemic to rapidly decrease

Common source Examples

anthrax, traced to milk or meat from infected animals

botulism, traced to soil-contaminated food

and cholera traced to fecal contamination of food and water

Point source epidemic

Duration

No

. of

case

s

Propagated Arise from infections being transmitted from one

infected person to another

Transmission can be through direct or indirect routes

Host-to-host epidemics rise and fall more slowly than common source epidemics

Propagated epidemic

Duration

No

. of

case

s

Examples of propagated epidemics

tuberculosis

whooping cough

Influenza

measles

Disease transmission Disease transmission usually occurs by

direct, person-to-person contact (e.g., STDs)

fomite-borne (e.g., Hepatitis A spread by a contaminated eating utensil)

vehicle-borne (e.g., HIV/AIDS spread through needle sharing drug users)

vector-borne transmission (e.g., Malaria spread through mosquitoes)

Mixed epidemics Occurs when a common source epidemic is followed

by person-to-person contact and the disease is spread as a propagated outbreak

Example – Shigellosis occurred among a group of 3000 women attending a wedding festival. Over the next few weeks, subsequent generations of shigella cases spread by person-to-person transmission from festival attendees.

Concepts and principles of case as used in epidemiology A case is a person who has been diagnosed as having a

disease, disorder, injury, or condition