Final answer:
Viruses typically infect only one host species due to the specificity of their surface molecules known as viral receptors. However, certain viruses can jump the species barrier and infect different species by undergoing genetic changes or mutations that allow them to bind to new receptors in a new host species.
Step-by-step explanation:
Viruses typically infect only one host species because they are very specific in terms of the hosts and cells they can infect. They have specific surface molecules known as viral receptors that allow them to attach to the host cell surface. The chemical differences in these receptors among hosts mean that a virus that infects one species may not be able to infect another species. However, there are certain viruses that are able to jump the species barrier and infect different species of organisms. This can happen when the virus undergoes genetic changes or mutations that allow it to bind to new receptors in a new host species. These changes occur randomly in the viral reproduction cycle, and if a virus with new binding properties comes into contact with a suitable host, it can spread quickly.