Viruses and viral infections have affected human history. Epidemics caused by viruses began when human behaviour changed during the Neolithic period, around 12,000 years ago. Previously hunter-gatherers, humans developed more densely populated agricultural communities, which allowed viruses to spread rapidly and subsequently to become endemic. Viruses of plants and livestock also increased, and as humans became dependent on agriculture and farming, diseases such as potyviruses of potatoes and rinderpest of cattle had devastating consequences. Smallpox and measles viruses are among the oldest that infect humans. They first appeared in humans in Europe and North Africa thousands of years ago, having evolved from viruses that infected other animals. Influenza pandemics have been recorded since 1580.
There are an estimated 1031 viruses on Earth. Although scientific interest in them arose because of the diseases they cause, most viruses are beneficial. They drive evolution by transferring genes across species, play important roles in ecosystems, and are essential to life.