SARS-CoV-2 virion A virus is a tiny infectious agent that reproduces inside the cells of living hosts. When infected, the host cell is forced to rapidly produce thousands of identical copies of the original virus. Unlike most living things,<unk>  do not have cells that divide; new<unk>  assemble in the infected host cell. But unlike simpler infectious agents like prions, they contain genes, which allow them to mutate and evolve. Over 4,800 species of<unk>  have been described in detail out of the millions in the environment. Their origin is unclear: some may have evolved from plasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria.<unk>  are made of either two or three parts. All include genes. These genes contain the encoded biological information of the virus and are built from either DNA or RNA. All<unk>  are also covered with a protein coat to protect the genes. Some<unk>  may also have an envelope of fat-like substance that covers the protein coat, and makes them vulnerable to soap. A virus with this "viral envelope" uses it—along with specific receptors—to enter a new host cell.<unk>  vary in shape from the simple helical and icosahedral to more complex structures.<unk>  range in size from 20 to 300 nanometres; it would take 33,000 to 500,000 of them, side by side, to stretch to .<unk>  spread in many ways. Although many are very specific about which host species or tissue they attack, each species of virus relies on a particular method to copy itself. Plant<unk>  are often spread from plant to plant by insects and other organisms, known as vectors. Some<unk>  of humans and other animals are spread by exposure to infected bodily fluids.<unk>  such as influenza are spread through the air by droplets of moisture when people cough or sneeze.<unk>  such as norovirus are transmitted by the faecal–oral route, which involves the contamination of hands, food and water. Rotavirus is often spread by direct contact with infected children. The human immunodeficiency virus, HIV, is transmitted by bodily fluids transferred during sex. Others, such as the dengue virus, are spread by blood-sucking insects.<unk> , especially those made of RNA, can mutate rapidly to give rise to new types. Hosts may have little protection against such new forms. Influenza virus, for example, changes often, so a new vaccine is needed each year. Major changes can cause pandemics, as in the 2009 swine influenza that spread to most countries. Often, these mutations take place when the virus has first infected other animal hosts. Some examples of such "zoonotic" diseases include coronavirus in bats, and influenza in pigs and birds, before those<unk>  were transferred to humans. Viral infections can cause disease in humans, animals and plants. In healthy humans and animals, infections are usually eliminated by the immune system, which can provide lifetime immunity to the host for that virus. Antibiotics, which work against bacteria, have no impact, but antiviral drugs can treat life-threatening infections. Those vaccines that produce lifelong immunity can prevent some infections.
Introduction to viruses
Viral pathogenesis
Human genetic enhancement
Xenobiotic
Human reproduction
Toxicant
Tumor initiation
Social immunity