Pathogens can be microorganisms such as bacteria, viruses, fungi, or parasites. They can also be prions, which are infectious proteins. Pathogens can spread through contact with an infected person or animal, through contaminated food or water, or through the air. Some pathogens can also be spread through contact with soil or surfaces that have been contaminated with the pathogen.

Pathogens cause disease by damaging cells and tissues in the body. They can also produce toxins, which are harmful chemicals that can cause a variety of symptoms. The severity of the disease caused by a pathogen depends on the type of pathogen, the person's immune system, and the amount of exposure to the pathogen.