These are organisms which are built for survival. They survive because they have evolved so as to generally go undetected. Although it s true that your Immune System can control some parasitic growth, it s generally unequipped to handle organisms which are multicellular and larger than most bacteria and viruses. Some parasites have even learned how to successfully invade the Immune System, by incorporating themselves around macrophages, suppressing immune responses, and releasing different antigens, so that the parasite, in effect, presents a moving target to the host's immune system. And, making their detection even more improbable, parasites generally spread throughout your body in very sophisticated ways, occupying different sites at various life stages, varying their size, shape and cellular composition in ways and degrees which are designed to operate beyond the Immune System s detection and eradication abilities.
Parasites are more common than most people realize, and they are easily contracted.
Protozoan are organisms that live in the blood, tissues and intestines. They are usually very small, generally microscopic, capable of infecting every tissue in the body. Malaria is the most common protozoan disease. Although exceptionally small, these parasites may remain active in the human body for an entire life time, causing multiple complications and revisitations, such as in malaria fever. They re generally flat and oval in shape.