In addition to "person" characteristics (or the "who"), the frequency of disease is described by the place of occurrence (or the "where"). For example, certain parasitic diseases such as malaria and schistosomiasis are known to occur in tropical areas. Other diseases may occur frequently in certain geopolitical entities; for example, gastroenteritis outbreaks often occur in communities with lax standards for water pollution. A third parameter that assists in defining disease patterns is time (or the "when"). Incidence rates may be tracked over a period of days or weeks (e.g., epidemics of infectious disease) or over an extended period of years (e.g., trends in the cancer death rate).