Parasites are organisms that live off other organisms, or hosts, to survive. Some parasites don’t noticeably affect their hosts. Others grow, reproduce, or invade organ systems that make their hosts sick, resulting in a parasitic infection. Parasitic infections are a big problem in tropical and subtropical regions of the world. Malaria is one of the deadliest parasitic diseases. Parasitic infections can also occur in the United States. Common parasitic infections found in the United States include: Trichomoniasis is a sexually transmitted infection caused by a parasite that often produces no symptoms. In some cases, it may cause itching, redness, irritation, and an unusual discharge in your genital area.