A virus is a non-living, acellular particle that can only reproduce inside a living host cell, so all viruses are obligate intracellular parasites. A virus particle (virion) consists of nucleic acid, either DNA or RNA, surrounded by a protein coat called the capsid. Some viruses, such as influenza and HIV, also have a lipid envelope taken from the host cell surface membrane, studded with viral glycoproteins.Viruses have no cytoplasm, ribosomes or enzymes for metabolism of their own. To reproduce, a virus attaches to specific receptor molecules on a host cell and its nucleic acid enters the cell. The virus then uses the host cell's ribosomes, ATP and nucleotides to make new viral nucleic acid and proteins. New virions are assembled and released, often destroying the host cell (the lytic cycle). Some viruses instead insert their genome into the host DNA and stay dormant for a time (the lysogenic or latent stage).
Because each virus binds only to particular receptors, viruses are specific to certain host species and tissues. Influenza infects cells lining the respiratory tract, while HIV infects T helper cells.Viral diseases of humans include influenza, measles, the common cold, HIV/AIDS and COVID-19. Plants also suffer viral diseases, such as tobacco mosaic disease. Because viruses lack the cell walls, ribosomes and metabolic pathways that antibiotics target, antibiotics have no effect on viruses.
Because each virus binds only to particular receptors, viruses are specific to certain host species and tissues. Influenza infects cells lining the respiratory tract, while HIV infects T helper cells.Viral diseases of humans include influenza, measles, the common cold, HIV/AIDS and COVID-19. Plants also suffer viral diseases, such as tobacco mosaic disease. Because viruses lack the cell walls, ribosomes and metabolic pathways that antibiotics target, antibiotics have no effect on viruses.