Any measurement in which the number of individuals in a study population that display a specified parasitic infection at a point in time or develop a specified parasitic infection within a determined period of time are compared to the total number of individuals in the study population. Often expressed as a percentage but could be expressed as a ratio. A parasitic infection is invasion and multiplication of parasites in body tissues, especially that causing local cellular injury due to competitive metabolism, toxins, intracellular replication, or antigen-antibody response. A parasite is an organism that grows, feeds, and is sheltered on or in a different organism while contributing nothing to the survival of its host and possibly causing damage to that host.