true or false

This depends on the type of disease you are talking about. In the case of a viral infection, this is true. A viral infection begins with the virus entering a person is body.

For most bacterial infections, this is also true. Although, this is not always true, as there are many forms of bacteria already within the body doing various good functions, such as E. Coli in your lower intestines.

In general, this statement is true, but there are exceptions.