The most obvious reason zoonoses are important is because they make animals and humans sick. They are also becoming ever more important due to increasingly rapid changes that are occurring in our environment (see the section "Factors in Disease Transmission and Drivers of Emerging Zoonoses").
However, there are also other activities not specifically affecting the environment that are unique to modern humans that are playing a role—not just in how sick we get from them or how they spread, but also in how they are used and misused. One example is the bioweaponization of certain zoonotic diseases. Although a multilateral agreement was ratified in 1975 by countries to stop biological warfare programs, many countries continue to develop them. The majority of these biological agents are zoonotic pathogens. Some examples that have been used include anthrax, brucellosis, Ebola and Marburg viruses, plague, rabies, and quite a few others. Perhaps even more importantly, terrorists are using an even wider variety of agents that are sometimes more difficult to track than those developed by governments. Although it may be harder for individual terrorists to procure and prepare such microbes, they don't have to be amenable to use on a battlefield and so in that sense it is easier.
A second example of a human activity that doesn't seem as scary at first glance is xenotransplantation. Because the demand for human-derived organs and tissues for transplantation has exceeded the supply, people are looking toward xenotransplantation as an alternative. Xenotransplantation refers to tissues derived from nonhuman animal sources. Although this offers possible hope for those waiting for transplants, it also opens the door for new zoonotic diseases and may be an ethical issue for many.
Yet another human activity is global trade. This is not as recent a development as the first two as global trade has been occurring for centuries. Still, the quantity and intensity of trade have exceeded the ability to properly monitor possible transmission of zoonoses. Intensive agricultural practices such as increasingly large herds that are raised in crowded conditions, as well as the trade of wild animals for food have increased the likelihood of also "trading" diseases. More will be discussed on this topic in the next section.
