Trypanosoma cruzi infects 11-12 million people in South and Central America and is infective to about 100-150 species of wild and domesticated mammals. It is not at all certain how many of these act as reservoirs of human infection but the armadillo is very important as in this host the infections are long-lived. The vectors are bugs belonging to the family Reduviidae of which three genera are important in the spread of the human disease. When the bug takes up infected blood the trypanosomes multiply in the epimastigote form in the hind gut and infective or metacyclic forms are passed out with the faeces. These infect the human host if they are rubbed into the bite, another wound or the conjunctiva of the eye. Within the human host, the trypanosomcs enter various cells, particularly macrophages, muscle and nerve cells, where they round up and multiply in the amastigote form.
The amastigotes develop into trypomastigotes that either enter new cells or are taken up when a vector feeds. The disease is called Chagas disease and takes various forms depending on where the amastigotes develop, the most serious consequences being cardiac failure due to parasites in the heart muscles or the loss of the nervous control of the alimentary canal due to parasites in the nervous system.