Protozoa that dwell in a human's gut are often transmitted to another human via a fecal-oral pathway (for example, contaminated food or water or person-to-person contact). Some protozoans can be transmitted by insects. Others cannot be transmitted directly from one person to another but instead must go through an intermediate host first. People become infected with protozoans when they eat food or drink beverages that have been contaminated with feces of someone who has a parasitic infection. The eggs or larvae of the protozoan may also be found in contaminated water. A person can become infected with more than one type of protozoan at a time. He or she might be exposed to one type of organism while eating a meal prepared by someone with another type of infection. The most common types of protozoans that affect humans are giardia and amoebae. Both of these organisms cause gastrointestinal problems for many people but can also lead to death if not treated promptly. Other types of protozoans that can infect humans include plasmodium, toxoplasma, and cryptosporidium. Many species of animals are susceptible to protozoal infections. They include birds, reptiles, amphibians, mammals, including humans.