Dividing parasites into microparasites and macroparasites. This is a fundamental distinction.

Is useful to distinguish small parasites, microparasites, such as bacteria, viruses and fungi, and large parasites, macroparasites, such a flukes and tapeworms.
Microparasites reproduce in the host, are short lived, are often lethal and often result in immunity for the survivors.
Macroparasites do not reproduce in the host, are long lived, often have complex life cycles - think tapeworms - and do not result in immunity.
For microparasites the spread of desease can be though of as numbers that are uninfected, infected, immune or dead. For macroparasites the spread of diseases is thought of as the change in the parasite burdon across hosts.