Protozoa are often spread through which of the following?

Protozoa are primarily spread through contaminated water, which is a significant means of transmission for many protozoan parasites. For instance, organisms such as Giardia lamblia and Entamoeba histolytica can be transmitted when humans consume water that has been contaminated with feces containing cysts of these parasites. This highlights the crucial role of sanitation and water quality in preventing diseases caused by protozoa.

While it is true that some protozoa can be spread by insect bites, particularly those that cause diseases like malaria (spread by Anopheles mosquitoes) or leishmaniasis (spread by sandflies), the question specifically asks about the common routes of transmission, where contaminated water remains one of the most prevalent globally. Airborne particles and direct contact with soil are not common transmission routes for protozoa; this limits their relevance in the context of how these organisms typically spread.