Final answer:
Viruses that transition from animals to humans must mutate to bind to human cell receptors and must come into direct contact with humans, particularly in environments where species intermingle closely, potentially leading to the rapid spread of disease among humans.
Step-by-step explanation:
Viruses that jump from animals to humans have two main characteristics that enable them to become zoonotic. First, a mutation must occur in the virus which allows it to bind to cell receptors of the new host. Since viruses have smaller amounts of DNA or RNA, they reproduce quickly, and mutations are a common product of their rapid replication cycle. These mutations can alter their surface proteins to the extent that they are able to interact with the receptors on human cells. Second, the virus must come into direct contact with the new host for the infection to take place.
In settings such as poultry and swine farms, where animals and humans are in close proximity, the possibility of cross-species transmission is higher. This is crucial for zoonotic diseases to emerge. Once a virus overcomes the species barrier, it can potentially spread rapidly among the human population. Thus, monitoring of emerging viruses is critical in preventing large-scale outbreaks.