The four different categories of infectious agents are bacteria , viruses , fungi , and parasites .

1. Bacteria - You can become exposed to bacteria from other people, the environment, or by eating or drinking contaminated food or water.

2. Viruses - A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism.

3. Fungi - Fungal infections are common throughout much of the natural world. In humans, fungal infections occur when an invading fungus takes over an area of the body and is too much for the immune system to handle. Fungi can live in the air, soil, water, and plants.

4. Parasites - Parasitic infections can be spread in a number of ways. For example, protozoa and helminths can be spread through contaminated water, food, waste, soil, and blood. Some can be passed through sexual contact.