A parasite is a creature that lives on or in the host and obtains its sustenance from or at the cost of the host. Protozoa, helminths, and ectoparasites are the three primary types of parasites that may cause disease in humans. Entamoeba histolytica is a kind of protozoa. It can invade the human body and cause amoebiasis. This disease is usually found in countries where clean water is not available for drinking purposes. Parasitic diseases are caused by organisms that live within their hosts and benefit from them somehow. Most parasitic infections are treated with antibiotics. However some parasites are able to resist antibiotic treatment. Infections caused by these parasites are called antibiotic-resistant infections. There are many different kinds of parasites that can infect people. Some examples are Giardia lamblia, Toxoplasma gondii, and Trypanosoma cruzi. Someone who has been exposed to any one of these parasites but survives the infection is said to have acquired passive immunity. The immune system of someone with passive immunity will be prepared if another person with similar antibodies gets infected with one of these parasites later on. That person's immune system will be able to fight the infection because it has been "prepared" by being exposed to these antibodies before. Someone who does not have protective antibodies but encounters one of these parasites for the first time will become susceptible to the infection.