Some other problems include food-borne illness and zoonosis, any disease or infection that is naturally transmissible from vertebrate animals to humans and vice versa, such as trichinosis, echinococcosis, and toxoplasmosis. Moreover, most wild animals can tolerate their parasite burdens fairly well, but crowdedness and malnutrition could subject infected herds to quick extinction unless a means of control of their parasites can be established in the near future. Silently suffering, domesticated animals and birds are subject to a wide variety of parasites often in greater numbers than in humans for the reason that they are usually confined to the same pastures, pens, or farms, so that the infective stages of parasites turn out to be exceedingly dense in the soil, and the burden of parasites within each host grows to be overwhelming. In company with their bacteria, fleas destroyed a third of the European population in the seventeenth century. Some serious parasites such as malaria, schistosomiasis, and African sleeping sickness have forward incalculable millions to their graves. All through history, the worldwide prevalence of selected parasitic diseases shows that there are more than enough existing infections for every living person to have one. There are three main classes of parasites: protozoa, helminths, and arthropods. A parasite is an organism that lives on or in a host organism and acquires its food from or at the expense of its host. Parasitology is an interesting field of biology, and parasites have been the subjects of some of the most exciting discoveries among infectious diseases.

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