Change in incidence of an infective disease is best judged in relation to incidence in the past as reflected in monthly incidence records; Mild incidence will be considered if:

1-3 new cases/100,000/week occur The following terms are used to express various grades of incidence and prevalence of communicable diseases in a community. 1. Mild incidence: When less than 3 new cases per week are repoed in 100,000 population. 2. Considerable incidence: When 3 to 5 new cases/ 100,000/week occur. 3. Heavy incidence: When 5 to 10 new cases/100,000/ week are repoed. 4. Epidemic: When the number rises to ten or more 100,000/week. 5. Outbreak: When there is sudden repoing of a large number of new cases, the population having been absolutely free earlier. Food poisoning and cholera often breakout suddenly. 6. Endemic: When the infectious agent has taken a foothold in a population which is naturally and, paially protected because of occurrence of the disease over a period. Examples are typhoid, diphtheria and infective hepatitis. When the incidence rises due to changes in the agent, host or environment factors, it becomes an epidemic. 7. Sporadic: When only isolated cases are repoed here and there, now and then. This may happen in case of meningitis and poliomyelitis. 8. Pandemic: When an epidemic appears simultaneously or successively in more than one country, e.g., the swine Flu pandemic 2009.