There are five main pathways how parasites enter our body:
Via food or water which are sources of the roundworms, amoebae, Giardia lamblia, Entamoeba histolytica, Cryptosporidium, Enterobius Vermicularis and many other parasites.
Via a vector
Mosquito can be a carrier of dog heartworm, filaria, malaria;
Flea can be a carrier of dog tapeworm;
Common housefly transmits amoebic cysts;
Sand fly carries leishmaniasis.
Via sexual contact. Partners can transmit trichomonas, Giardia, amoebae...
Through the nose and skin where pinworm eggs and Toxoplasma gondii can be inhaled from contaminated dust. Hookworms, schistosomes, and Strongyloides can penetrate exposed skin and bare feet.
Blood transfusion can spread blood parasites