Bacteria: These are single-celled organisms that can be beneficial or harmful. Harmful bacteria (pathogenic) can cause diseases like strep throat or tuberculosis.
Viruses: These are tiny infectious agents that replicate only inside the living cells of an organism. They can cause diseases ranging from the common cold to more severe illnesses like HIV/AIDS.
Fungi: This is a group of eukaryotic organisms that includes yeasts, molds, and mushrooms. Some fungi are pathogenic and can cause diseases such as athlete's foot or ringworm.