Transmission of E. histolytica from man to man occurs through its encysted stage and infection is effected through the ingestion of these cysts in food or drinks. It is to be noted that mature quadrinucleate cysts are the infective forms of this parasite.
The contamination of food and drinks is occurred by:
i. Faecal contamination of drinking water, vegetables and food are the primary causes of infection.
ii. The water supply may be contaminated with infected faeces and infection may occur through such contaminated drinking water which give rise to epidemics.
iii. The infection is effected by swallowing quadrinucleate cysts with uncooked vegetables and fruits.
iv. The cysts of E. histolytica have been found in the droppings of houseflies and cockroaches which also serve as a source of infection.
v. Raw vegetable is also another source of infection.
vi. In many countries human faeces is used as fertiliser and thus roots and leaves of plants remain contaminated with viable cysts.