Viruses

Viruses are made up of a piece of genetic code, such as DNA or RNA, and protected by a coating of protein. Once you’re infected, viruses invade host cells within your body. They then use the components of the host cell to replicate, producing more viruses.

After the replication cycle is complete, these new viruses are released from the host cell. This usually damages or destroys the infected cells.

Some viruses can remain dormant for a time before multiplying again. When this happens, a person appears to have recovered from the viral infection but gets sick again.

Bacteria

Bacteria are microorganisms made of a single cell. They are very diverse, have a variety of shapes and features, and have the ability to live in just about any environment, including in and on your body. Not all bacteria cause infections. Those that can are called pathogenic bacteria.

Your body can be more prone to bacterial infections when your immune system is compromised by a virus. The disease state caused by a virus enables normally harmless bacteria to become pathogenic.

Fungi

There are millions of different fungal species on Earth. Fungi can be found just about everywhere in the environment, including indoors, outdoors, and on human skin. They cause infection when they overgrow.

Fungi cells contain a nucleus and other components protected by a membrane and a thick cell wall. Their structure can make them harder to kill.

Some new strains of fungal infections are proving to be especially dangerous, such as Candida aurus, and have prompted more research into fungal infections.

Parasites

Parasites are organisms that behave like tiny animals, living in or on a host and feeding from or at the expense of the host. Though parasitic infections are more common in tropical and subtropical regions, they can occur anywhere.

Three main types of parasites can cause disease in humans. These include:

• protozoa, which are single-celled organisms that can live and multiply in your body
• helminths, which are larger, multi-celled organisms that can live inside or outside your body and are commonly known as worms
• ectoparasites, which are multi-celled organisms that live on or feed off your skin, including some insects, such as ticks and mosquitos

They can be spread in several ways, including through contaminated soil, water, food, and blood, as well as through sexual contact and insect bites.