Final answer:
The most common source of human infection in the world is parasites.
Step-by-step explanation:
Infectious diseases can be caused by various types of pathogens, including bacteria, viruses, parasites, and prions. However, the most common source of human infection in the world are parasites. Parasites can be transmitted to humans through the consumption of contaminated food or water, or by handling fecal-contaminated soil or animal waste. Examples of common parasitic infections in humans include malaria, giardiasis, and toxoplasmosis.