Cryptosporidiosis (or “Crypto” for short) is a disease that causes watery diarrhea. It is caused by microscopic germs—parasites called Cryptosporidium. Although Crypto can affect all people, some groups are likely to develop more serious illness. For people with weakened immune systems, symptoms can be severe and could lead to severe or life-threatening illness. Examples of people with weakened immune systems include those with HIV/AIDS; those with inherited diseases that affect the immune system; and cancer and transplant patients who are taking certain immunosuppressive drugs.