According to the Centers for Disease Control and Prevention (CDC), there are over 1.5 million distinct species of fungus on Earth, 300 of which cause sickness in humans. Fungal illnesses are frequently caused by ordinary fungus prevalent in the environment. Most fungi are harmless, however others can be damaging to one's health. Some fungal infections can be cured with medicine while others require medical attention. Fungi are classified as "opportunistic" pathogens because they don't need a human host to survive and spread within its environment. This means that anyone may get them, but not all people who get them will become sick. Risk factors for getting sick include weak immune systems, such as those resulting from cancer treatments, AIDS, or other diseases or disorders that reduce the body's resistance to infection; people who use drugs or steroids that suppress their immune system; premature babies; and those who have been organ transplants or received bone marrow transplants. The two main types of fungal infections are dermatophytosis (skin disease) and candidiasis (yeast infection).