Infective agents of ultramiscroscopic size, filter-passing, intracellular, obligatorily parasitic, cultivable only on living cells as opposed to the classical inanimate culture media, and resistant to many antibiotics and chemical agents lethal to bacteria. Viruses are measured by their size in nanometres, vary from 10 to 450 nm, and consist of a core of nucleic acid (RNA or DNA) surrounded by a protein coating. They initiate infection by entering body cells, within which the infecting particles (elementary bodies) rapidly multiply and spread to further cells, with or without the death of the affected cells. Virus-infected cells may show large cytoplasmic or nuclear acidophilic (less often basophilic) bodies (‘inclusion bodies’) connected with virus multiplication, their site and size being diagnostic features of the various species. Other aids to identification are the cytopathic effect (CPE)—i.e., the nature and degree of virus effect on living cells in culture, the creation of haemagglutinating or complement-fixing antigens in such cultures, the adsorption of red cells to the infected culture cells (haemadsorption) and disturbance of the metabolism of these cells.
Infectious microorganisms that are found in all life forms, including hu¬ mans, animals, plants, fungi, and bacteria. Each viral unit, or virion, consists of a strand of ribonucleic acid (RNA) or deoxyribonucleic acid (DNA). No virus contains both RNA and DNA, a characteristic that distinguishes viruses from all other cells. The virion is enclosed in a protein shell of one or two layers, called a capsid. Some viruses are coated with another layer called a viral envelope.
Submicroscopic organisms made up of complex proteins that can grow and multiply inside living cells and cause infectious diseases.
Among the tiniest recognized forms of infectious agents, the status of viruses as genuine living entities or as assemblages of molecules capable of self-replication under certain circumstances remains a topic of debate. Their singular function involves infiltrating the cells of other organisms, subsequently commandeering these cells to replicate their own copies. When outside living cells, viruses remain inactive.
A solitary virus particle, referred to as a virion, comprises an inner nucleus composed of nucleic acid, which can be either DNA or RNA, encased by one or two safeguarding protein coats known as capsids. Beyond the outer capsid, there might exist an additional layer known as the viral envelope, primarily comprised of protein. The nucleic acid consists of a sequence of genes that hold encoded directives for the replication of the virus.
Frequent viral ailments encompass the ordinary cold, influenza, chickenpox, cold sores, and warts. The human immunodeficiency virus is responsible for causing AIDS.