Chytrid is a type of fungus. There are many, many different varieties of Chytrid (over 1,000), with most feeding on decaying plant matter. A few forms actively parasitize living things. One species, Batrachochytrium dendrobatidis, is capable of parasitizing vertebrates. This species is responsible for many amphibian declines around the country, and has been linked to populations going extinct around the world.
When an amphibian is infected with Chytrid, the fungus attacks the cells in the outer layer of skin, causing it to thicken. When this happens, the animal is considered to have chytridiomycosis. Amphibians exchange electrolytes and water through their skin. As chytridiomycosis progresses if left untreated, these electrolyte levels ultimately become unbalanced, and the amphibian dies. Chytrid does not kill every amphibian it infects. Some species, such as clawed frogs and bullfrogs, appear to be able to carry chytrid without showing symptoms. Bullfrogs are commonly farmed for frog legs, while clawed frogs have been used extensively in the medical field. It is thought that these species may have contributed to Chytrid spreading around the globe.
Chytrid requires relatively cool, wet conditions to survive and spread. Therefore, it's important to properly disinfect any items that could potentially spread chytrid by elevating the temperature (100F for 4 hours) and allowing the items to completely dry out. Cooking leaf litter or wood in the oven at a low setting is a popular way to do this in the hobby. Non-porous surfaces can be disinfected with a 1% bleach solution.
To prevent Chytrid, quarantine and test any new animals for 60-90 days. Watch for any visible symptoms, such as discoloration, odd behavior, or problems shedding. Remember that animals which have Chytrid do not always show symptoms, and that many symptoms the animal may exhibit could easily have other causes. It is impossible to diagnose Chytrid without testing, so don't jump the gun!
Chytrid is treatable in the vast majority of cases. A popular treatment prescribed by exotic vets is based on a series of baths in the drug itraconazole. For a couple years, using a specific type of Lamasil Athlete's Foot Spray was a common hobby remedy that seemed to work, but unfortunately this medication is no longer available in the USA. Amphibians can react to different medications and treatments in different ways - it's important to consult with a veterinarian experienced in treating amphibians. To find an exotics vet near you, use the Association of Reptile and Amphibian Veterinarians website.