A person can have low blood platelet count, or thrombocytopenia, because of genetic history or medical conditions, according to Mayo Clinic. Drugs such as those used in chemotherapy can also cause a low platelet count.
Some medical conditions that can lead to thrombocytopenia include leukemia and viral infections such as hepatitis C, according to Mayo Clinic. People with HIV can also have lowered platelet counts, as can those who are heavy alcohol users.
In some cases, the platelets are trapped in the spleen and can't be released into the circulatory system, Mayo Clinic explains. An enlarged spleen may be an indication that a person's platelet count is low. Pregnant women may experience a mild thrombocytopenia that goes away after they give birth. Some forms of the disorder are the result of autoimmune diseases, where the body is attacked by its own immune system.
In addition to viral infections, bacterial infections can also lower the platelet count, states Mayo Clinic. In this case, the bacteria destroy the platelets. Hemolytic uremic syndrome may be the result of an infection by the E. coli bacteria. This condition destroys both red blood cells and platelets. Tiny blood clots can also form in the body and take platelets out of circulation.