Bacteria

Bacteria are microscopic, single-celled organisms without a nucleus.

Streptococcus pyogenes

Pharyngitis (strep throat)

Viruses

Viruses are small infectious organisms—much smaller than a fungus or bacterium. They cannot reproduce on their own. They must invade a living cell and use that cell’s machinery to reproduce.

Varicella zoster

Chickenpox and shingles

Fungi

Fungi are neither plants nor animals. Their size ranges from microscopic to easily seen with the naked eye. They include yeasts, molds, and mushrooms.

Candida albicans

Vaginal yeast infections

Parasites

Parasites are organisms that survive by living inside another usually much larger organism (the host). They include worms and single-celled organisms called protozoa (which, unlike bacteria, have a nucleus).

Enterobius vermicularis (a species of pinworm)

Itching around the anus