People get sick when another organism, big or small, invades their body and infects them. These infectious agents come in all shapes and sizes, and all of them pose different threats to the human body. Some are microscopic, such as bacteria or viruses, which attack human bodies on the cellular level. Others are larger, like fungi, which are unicellular or multicellular organisms that grow on and feed off organic material, including humans. Finally, parasites such as tapeworms can find their way inside the human body and feed on blood and nutrients without killing their host.

Learn more about infectious agents and their impact on human health with this curated resource collection.