Viruses are distinct from all other biological entities on Earth because they lack one key feature of all living organisms: the ability to reproduce on their own.
Like living cells, viruses are comprised of genetic material protected by proteins, and sometimes membranes, but their genomes are vastly smaller than their bacterial counterparts — up to 100s of times smaller! With limited genomic capacity, a virus cannot reproduce on its own. Instead, viruses require a host cell to supply the building materials and machinery needed to make more viruses while the virus provides the genetic blueprint.
Because viruses are completely dependent on their hosts to replicate, they have evolved a vast arsenal of tricks for getting into host cells, co-opting those cells to make more viruses, and then escaping to infect more cells. Viruses have developed these tricks through millions of years of evolutionary battle with their hosts — the host evolves to avoid the virus and the virus evolves countermeasures to evade host defenses and gain access to its cellular machinery. After so much back and forth, most host-virus interactions are very species specific and indeed, many viruses only infect specific kinds of cells within their hosts. Each virus has evolved a life cycle strategy to overcome its inability to reproduce on its own, and that strategy affects when and where disease outbreaks occur.
After figuring out how to invade and replicate in host cells, viruses also have to get from host to host. Some viruses are spread via biting (rabies), sex (HIV), sneezing or coughing (influenza), or fecal contamination of drinking water (polio). WNV uses mosquitoes to carry it from bird to bird, birds being the hosts that it uses for amplification within the environment.