Contaminated water is one source of infection. Improperly washed or undercooked food is a common means of infection. Transmission from pets is another. Contact with another infected person is also a common route of infection. Travel can escalate the risks.
Antibiotics pose another problem because they interfere with the normal intestinal flora.
Treatment Since there are many types of parasites, each with its own particular life cycle and pattern, a few generalizations may simplify the rationale behind the different treatment strategies. First, one needs to understand that the parasite is a creature that depends on a host for survival, ergo its name. It leeches nutrients that the host needs in order to be healthy. Second, the parasite invades a bodily structure and inflicts damage to that structure so that healing requires both the elimination of the parasite and the regeneration or rejuvenation of the affected organs. So, one thing at a time.