Climate and sanitary conditions breed parasites, of course, and then they are spread throughout the world. International travel spreads them, as well as the return of armed services personnel from overseas. Within a country, parasites find a home in contaminated municipal and rural water supplies and in household pets, not to mention in farm animals. Finally, the increase of daycare centers has proven to be a transmission place. Parasites are larger than bacteria and viruses but usually so small that you cannot see them without a microscope.
Bacteria can also be parasitic.
The parasites are often harmful. Any number of them can infect your gastrointestinal tract. Most parasites produce similar symptoms. According to Ann Louise Gittleman, a certified nutritionist who has worked with parasite patients, a parasite infection can be manifested by any number of signs. These include constipation, diarrhea, gas and bloating, irritable bowel syndrome, joint and muscle aches and pains, anemia, allergies, skin conditions, nervousness, sleep disturbances, teeth grinding, and chronic fatigue.