A virus (from the Latin virus meaning toxin or poison) is a small infectious agent that can replicate only inside the living cells of other organisms. Most viruses are too small to be seen directly with a light microscope. Viruses infect all types of organisms, from animals and plants to bacteria and archaea. Viruses vary from simple helical and icosahedral shapes to more complex structures. Most viruses are about one hundred times smaller than an average bacterium. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity.
Viruses spread in many ways; plant viruses are often transmitted from plant to plant by insects that feed on sap, such as aphids, while animal viruses can be carried by blood-sucking insects. Influenza viruses are spread by coughing and sneezing. HIV is one of several viruses transmitted through sexual contact and by exposure to infected blood. Viruses can infect only a limited range of host cells called the "host range". Viral infections in animals provoke an immune response that usually eliminates the infecting virus. Microorganisms also have defences against viral infection, such as restriction modification systems which restrict the growth of viruses.
Viruses display a wide diversity of shapes and sizes, called morphologies. Generally viruses are much smaller than bacteria. Most viruses that have been studied have a diameter between 10 and 300 nanometres. To increase the contrast between viruses and the background, electron-dense "stains" are used. When virions are coated with stain (positive staining), fine detail is obscured. A complete virus particle, known as a virion, consists of nucleic acid surrounded by a protective coat of protein called a capsid. Viruses can have a lipid "envelope" derived from the host cell membrane.
Virally coded protein subunits will self-assemble to form a capsid, generally requiring the presence of the virus genome. Complex viruses code for proteins that assist in the construction of their capsid. Proteins associated with nucleic acid are known as nucleoproteins, and the association of viral capsid proteins with viral nucleic acid is called a nucleocapsid. The capsid and entire virus structure can be mechanically (physically) probed through atomic force microscopy
