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Rhizopus is a genus of fungi within the Zygomycetes class, a large grouping of fungi which are notable for their method of sexual reproduction. Fungi in the Rhizopus genus are often responsible for zygomycosis, an infection caused by colonization with fungi in the Zygomycetes class. These fungi also have some practical functions, and they can appear in the form of plant pathogens in some regions of the world. This makes them a rather diverse group.
Classically, Rhizopus fungi look like cotton candy, with a yellow to green color which darkens when the fungus begins to produce spores. These fungi can reproduce asexually or sexually by creating a zygospore, a fusing of two spores. This ability is unique to fungi in the Zygomycetes class. The spores often burrow into the surface of the substrate, making the mold difficult to remove, since it cannot simply be brushed off.
One of the most common forms of Rhizopus is bread mold, R. nigricans. These fungi also appear in fruit, especially stone fruit, and they can be found in vegetables, feces, and the soil. The fungus grows very rapidly, maturing within four days, as anyone who has watched a perfectly good peach rot almost before their eyes is well aware. This rapid growth is one of the reasons why Rhizopus can be quite a problem, as the fungus grows and produces spores so quickly that it is very hard to control.
The fungus needs a warm, moist environment to thrive. People can prevent or stave off Rhizopus infection by keeping food under refrigeration or in the freezer so that the spores never get a chance to grow. In the case of crops, it can be difficult to prevent the spread of the fungus once spores start to land. These fungi also have some uses; several species are used in the controlled fermentation of foods, especially in Asia, for example.
People can become infected with Rhizopus by eating contaminated food or inhaling the spores. In rare cases, people can also be infected as a result of puncture wounds. Once the spores get into the body, they mature and start to grow, spreading hyphae through the vascular system and causing a range of health problems. Zygomycosis usually appears near the area where the fungus was introduced to the body, but it can spread to regions like the brain. Antifungal medications can be used to treat the infection, although it may become problematic in someone with a compromised immune system.
You're right, rhizopus can cause really serious problems in humans -- in fact, it can sometimes be life-threatening. I work in a health clinic, and I remember we once had a girl come in with a serious rhizopus infection in her sinuses. It was so aggressive that the doctors decided to do surgery, because, if left untreated, a rhizopus infection can even spread to the brain!