Bacteria: Bacteria are microscopic, single-celled organisms. Examples: Streptococcus pyogenes (strep throat); Escherichia coli (urinary tract infection).

Viruses: A virus is a small infectious organism—much smaller than a fungus or bacterium—that cannot reproduce on its own; it must invade a living cell and use that cell's machinery to reproduce. Examples: Varicella zoster (chickenpox, shingles); Rhinovirus (common cold).

Fungi: Fungi are actually a type of plant. Yeasts, molds, and mushrooms are all types of fungi. Examples: Candida albicans (vaginal yeast infection); Tinea pedis (athlete's foot).

Parasites: A parasite is an organism, such as a worm or single-celled animal (protozoan) that survives by living inside another, usually much larger, organism (the host). Examples: Enterobius vermicularis (pinworm); Plasmodium falciparum (malaria).