 P 
Pandemic
An epidemic occurring over a very wide area (several countries or continents) and usually affecting a large proportion of the population.
Pathogenicity
The proportion of persons infected, after exposure to a causative agent, who then develop clinical disease.
Percentile
The set of numbers from 0 to 100 that divide a distribution into 100 parts of equal area, or divide a set of ranked data into 100 class intervals with each interval containing 1/100 of the observations. A particular percentile, say the 5th percentile, is
Period Prevalence
The amount a particular disease present in a population over a period of time.
PersonTime Rate
A measure of the incidence rate of an event, e.g.
Pie Chart
A circular chart in which the size of each ``slice'' is proportional to the frequency of each category of a variable.
Point Prevalence
The amount of a particular disease present in a population at a single point in time.
Population
The total number of inhabitants of a given area or country. In sampling, the population may refer to the units from which the sample is drawn, not necessarily the total population of people.
Predictive Value Positive
A measure of the predictive value of a reported case or epidemic; the proportion of cases reported by a surveillance system or classified by a case definition which are true cases.
Prevalence
The number or proportion of cases or events or conditions in a given population.
Prevalence Rate
The proportion of persons in a population who have a particular disease or attribute at a specified point in time or over a specified period of time.
Propagated Outbreak
An outbreak that does not have a common source, but instead spreads from person to person.
Proportion
A type of ratio in which the numerator is included in the denominator. The ratio of a part to the whole, expressed as a ``decimal fraction'' (e.
Proportionate Mortality
The proportion of deaths in a specified population over a period of time attributable to different causes. Each cause is expressed as a percentage of all deaths, and the sum of the causes must add to 100%.
Public Health Surveillance
The systematic collection, analysis, interpretation, and dissemination of health data on an ongoing basis, to gain knowledge of the pattern of disease occurrence and potential in a community, in order to control and prevent disease in the community.
