What Are Pathogens?
Pathogens—also known as infectious agents or germs—are microorganisms that cause infectious diseases. When these organisms enter and attack a host (a human or other living organism capable of carrying a pathogen), they can trigger symptoms and illness.
There are five main types of pathogens:
- Bacteria – Microscopic, single-celled organisms that come in various shapes such as rods, spirals, and spheres. While most bacteria are harmless or even beneficial, some can cause disease. Bacteria contain DNA but lack a true nucleus.
- Viruses – Much smaller than bacteria, viruses can only reproduce inside a living host cell. Once inside, they use the host’s cellular machinery to multiply, often damaging or destroying cells in the process.
- Fungi – These organisms can be single-celled (like yeast) or multicellular (like molds). Fungi have a nucleus and a thick cell wall and can survive outside a host. Some cause infections, particularly in damp or unclean environments.
- Protozoa – Single-celled organisms that live in water and often feed on other microbes. They can infect humans by multiplying inside the body, leading to diseases such as malaria.
- Helminths – Also known as parasitic worms, these are large, multicellular organisms that live inside the body and cause illness by feeding on the host’s nutrients.