Occurrence of Disease 。Incidence Fraction of a population that contracts a disease during a specific time. 。Prevalence Fraction of a population having a specific disease at a given time. Sporadic disease Disease that occurs occasionally in a population. Endemic disease Disease constantly present in a population. 。Epidemic disease Disease acquired by many hosts in a given area in a short time. 。Pandemic disease Worldwide epidemic. 。Herd immunity Immunity in most of a population. • Incidence Fraction of a population that contracts a disease during a specific time. • Prevalence Fraction of a population having a specific disease at a given time. • Sporadic disease Disease that occurs occasionally in a population. • Endemic disease Disease constantly present in a population. • Epidemic disease Disease acquired by many hosts in a given area in a short time. • Pandemic disease Worldwide epidemic. • Herd immunity Immunity in most of a population. Occurrence of Disease