A virus is a biological agent that reproduces inside the cells of living hosts such as cattle. When infected by a virus, an animals body is forced to produce many thousands of identical copies of the original virus, at an extraordinary rate. Viruses are very small and it might take 1,000,000 of them or more, side by side, to stretch 1 inch.
Viruses can be spread by blood-sucking insects, through the air by cattle when they cough or sneeze, spread through food or water contaminated by fecal matter of animals with the virus, through milk, and through mating.
Viral infections can cause disease, however they are usually eliminated by the immune system; cattle that are weak will have weak immune systems and are more vulnerable to viruses. Antibiotics kill bacteria and have no effect on viruses. Vaccines can produce an artificial immunity in cattle which can help prevent viral infections.