Millions of Americans become infected with parasites, and their symptoms frequently go unreported or are misdiagnosed. These tiny critters are commonly found in food and water. Seizures, blindness, pregnancy issues, heart failure, and even death can all result from an infection. Parasites are organisms that live in or on humans or animals for no significant part of their lives. Although parasites may cause some health problems for their hosts, they usually benefit the host by eliminating other organisms that would otherwise harm them. Some parasites may cause disease in their hosts but many more exist without causing any damage at all. Most parasites are bacteria or viruses. Others include: • Flukes - including liver flukes and intestinal flukes - that infect fish-eating birds and mammals, respectively. Liver flukes cause cancer by altering the host's liver tissue. Intestinal flukes cause diarrhea by blocking up the intestines with eggs. • Nematodes - including roundworms and hookworms - that infect humans. Roundworms are divided into two groups based on how they develop: plant- and animal-borne rounds. Hookworms are transmitted through the feces of an infected person or animal. They travel to the lungs where they mature before being coughed up and swallowed by a new host.