Parasitic fungi are members of the Fungi kingdom, which thrive by latching on to other organisms and taking nutrients from them. They are a type of symbiotic fungi, but they are not mutualistic because they do not tend to give anything of value back to their host in return for sustenance. Fungi can be separated into three distinct groups, based on how they interact with their hosts: mycorrhizal fungi work in cooperation with a living host, parasitic fungi attack a living host, and saprophytic fungi feed on a dead host.
Generally, these fungi are highly specialized, designed to attack one or a handful or particular species of plants or animals to survive. In some cases, however, the fungi may be quite general and able to adapt to integrate with a wide range of creatures. Parasitic fungi can be separated into two large groups — the Basidiomycetes and the Ascomycetes — with only a small handful of fungi existing outside of these two sets. Although not actually fungi, many people consider the Oomycetes to be in the parasitic group, because they share many properties with true fungi. These fungi-like parasites, which include those responsible for sudden oak death and potato blight, are some of the most destructive plant parasites on the planet.
Although parasitic fungi live on living hosts, many are adapted to transform into saprophytic fungi should they kill their host. In this case, they will find a healthy host, invade it, and drain its nutrients until the creature dies, and then continue to feed on the dead host, aiding in the process of decay. Some fungi will then go on to find a fresh host and repeat the cycle.
There are thousands of species of fungi that are parasitic, and some have adapted to be extremely specialized, almost to the point of absurdity. For example, there are types of Laboulbeniales, such as the obligate insect parasites, which are adapted to infest only the Harmonia axyridis beetle. They infest these beetles so pervasively that for years, entomologists believed the fungus was simply a characteristic of the beetle itself. Even more extreme examples of these fungi are adapted to infest only the left side of a specific fly.
One of the primary defenses against parasitic fungi are actually other fungi. Many of the Mycorrhizal fungi, which form mutualistic relationships with other organisms, protect those organisms against the parasitic variety invading their space. They do this in a number of different ways, including excreting natural antibiotics that kill off other fungi adapted to that particular host. Plants and some animals therefore have adapted to encourage the growth of the Mycorrhizal fungi as a way of protecting themselves from the damaging parasites.