Finally, these diseases tend to occur only in humans; The germs that cause these diseases tend not to live in the soil or in other animals. Dog And She Lived Happily Ever After poster. All four characteristics are true for childhood infectious diseases such as measles, rubella, mumps, whooping cough and smallpox. It is easy to understand why these four characteristics combine to make a disease tend to occur in epidemics. Germs spread rapidly and the symptoms occur so quickly that everyone in a local population quickly becomes infected and soon dies or will either recover or become immune.