Final answer:
The principal living reservoir of human disease is animals, from which zoonotic diseases can be transmitted to humans. Handling infected animals or coming into contact with their waste can lead to the spread of such diseases.
Step-by-step explanation:
The principal living reservoir of human disease comes from animals. Zoonotic diseases, or zoonoses, are diseases that can be transmitted from animals to humans and vice versa. The concentration of large numbers of people in urban areas has increased the spread of infectious diseases, particularly through direct or indirect contact with animals or contaminated objects. Numerous diseases are spread in this manner, which can involve handling infected animals or coming into contact with their waste products.
Some illnesses, for example, are spread by consuming food or water contaminated with animal feces, or through handling fecal-contaminated soil or a cat's litter box. This kind of contamination can contribute to diseases like toxoplasmosis. Moreover, the presence of rodents and their droppings in close quarters with humans and their food supplies can also lead to the spread of diseases, such as the plague in historical contexts.