Fungi (which is the plural form of fungus) are not considered as parasites following the strict definition of the term in the medical world. In fact, the term parasite in this domain is usually restricted to the organisms belonging either to the protozoan parasite or the helminth groups. However, other living organisms might be able to establish a negative relationship with a host, which is called parasitism. This type of biological relationship implies that the organism parasitizing the host benefits from the relation while the host suffers from negative effects. In that case, the organism whose practices parasitism might sometimes be called a parasite, but the use of the term parasite here has a much broader sense than what we call parasite in the medical parasitology field. Following this broader meaning, fungi can be sometimes called parasites, as some of them are able to establish a parasitism relationship with other living organisms like human.
Now, let’s talk a little bit more about fungi. The biological kingdom of the Fungi is composed of eukaryotic cells, which is the same type of cells that compose parasites, animals and plants (by opposition to the prokaryotic cell of bacteria), meaning that their cells have a nucleus protecting their genetic materials and mitochondria responsible of the production of energy. This biological kingdom includes unicellular organisms, such as yeasts and molds that are considered as microorganisms, and multicellular organisms, such as mushrooms. They are present in large amounts worldwide. Actually, it has been estimated that there are 1.5 million different species of fungi all around the world. Furthermore, fungi play an important role in the nutrient cycles in nature by being key players in the decomposition of organic matters.
However, depending on the species involved, fungi can infect and parasitize plants, animals or humans. Among all the fungi, only 300 species are able to infect humans at some point. The medical branch that is interested by infections with fungi is called medical mycology. When they parasitize humans, fungi cause an infection that is susceptible to be, in some rare cases, severe and even fatal if left untreated. However, most often, fungi are not very dangerous for the human health and cause only mild skin, nails or vaginal infections. These infections are more annoying than anything else and might require a treatment in order to get rid of the fungus involved. It is interesting to note that fungal vaginal infections by yeasts are mostly occurring in women who have took antibiotics. In fact, the vagina contains naturally a small population of yeasts that is controlled by a balance with the population of bacteria. While taking some antibiotics, a proportion of the population of bacteria naturally present in the vagina might die. In that case, the population of yeasts is able to take advantage of the unbalance in order to proliferate and establish an infection.
Infections with fungi are susceptible to happen to anyone, but are quite more frequent for people with weakened immune system, such as people infected by AIDS. People taking some specific types of medication, such as cancer chemotherapy and corticosteroids, have also a weakened immune system and are then more susceptible to acquire fungal infections. Furthermore, some fungi are susceptible to cause only mild infection in healthy people, but can be life-threatening for people with weakened immune system. Consequently, many fungal infections are considered as opportunistic infections, meaning that they are harmless for healthy people, but are susceptible to take advantage of the most vulnerable people in order to establish an infection. It is interesting to note that the previously mentioned vaginal yeast infection occurring sometimes in women who took antibiotics can also be considered as an opportunistic infection.
Source: Centers for Disease Control and Prevention