Patterns in Epidemiology of Sexually Transmitted Diseases in Human Populations

Infectious diseases exhibit a lot of interesting patterns when they spread in host populations. Detailed data are especially available on infectious diseases in human populations. Many typical infectious diseases of childhood, like measles and rubella, show clear seasonality with period of one year [2, 7, 8, 9, 12]. In addition to such annual patterns, they also show periodicity with period more than one year, eg., two years, five years, etc. The period of the same disease may be different in different places and in some places there may be chaotic patterns rather than periodic patterns. Moving focus from such childhood diseases to those of adulthood or adolescents, sexually transmitted diseases (or STDs for short, hereafter) also have interesting patterns although they cannot be clearly observed like childhood diseases. In most studies of pattern formation, some interesting pattern is generated similar to a shadow of some other preceding pattern. In the analysis of STDs, we may find cases where a pattern of more interest exists behind the apparently observed pattern. By the analysis of patterns observed in incidences of STDs, we can find patterns of human sexual behavior, the detail of which is usually hidden although questionnaire surveys can sometimes shed light on it. Some patterns of STDs are common to infectious diseases of childhood. The growth of the number of infected people is exponential at the beginning phase of spread. Another typical pattern is one observed in incidences according to the age of hosts. We first briefly show descriptive epidemiology of STDs in Japan below. The data are collected on the outpatients who visited urological, dermatological, obstetric or gynecological clinics in 1999. After reviewing some observed patterns in epidemiology, we try to explain these patterns by mathematical models of infectious diseases.