The answer to this can be found in Chapter 11 of Guns, Germs, and Steel. Specifically, it can be found beginning on p. 202 of the paperback edition of the book.
In this chapter, Diamond is discussing how epidemic diseases came to us through animals. He is also talking about how they were important for Europeans since they killed many people in the places that the Europeans conquered. In this discussion, Diamond differentiates between infectious diseases that come in epidemics and those that come in a “steady trickle of cases.” The diseases that come as epidemics are much more dangerous to populations that are exposed to them for the first time. There are four main characteristics of epidemic diseases that the regular infectious diseases do not share. They are:
- Speed of transmission. Epidemic diseases spread quickly among people. This means that everyone in a population gets exposed to them in a very short time.
- Severity of the disease. These diseases are “acute.” People who get them do not keep suffering from them on a chronic basis. They either recover or they die.
- Immunity. Those people who do not die from the diseases become immune to them for life.
- Where the diseases can live. Epidemic disease microbes tend to live only in people. They cannot live in the soil or in animals other than people.