What Are Viruses?
Viruses are the smallest of the microbes and are essentially just a bundle of genetic material (DNA or RNA) carried in a coating of proteins called a capsid. Viruses are dormant until they are in contact with living cells, which they depend on to multiply. They invade living cells with the sole intention of replicating through hijacking the cell's reproductive machinery by way of feeding the cell its own genetic instructions to replicate more of the same virus. Viruses are therefore parasitic by nature.
They are also "picky" by nature - they only invade specific types of cells depending on the virus type. This specificity means, for example, that a bacteriophage virus will infect a bacterial cell but will remain inactive when in contact with animal or plant cells, because the viral proteins have to match with receptors on the host cell's wall. Plant viruses, like the tobacco mosaic virus, will infect only plants. Even more specific, polio viruses only infect spinal nerve cells, and HIV viruses only infect certain types of white blood cells in humans. The illustration below shows the infection process for an virus type called paramyxovirus that affects and causes a number of diseases in humans and other animals:
They are also "picky" by nature - they only invade specific types of cells depending on the virus type. This specificity means, for example, that a bacteriophage virus will infect a bacterial cell but will remain inactive when in contact with animal or plant cells, because the viral proteins have to match with receptors on the host cell's wall. Plant viruses, like the tobacco mosaic virus, will infect only plants. Even more specific, polio viruses only infect spinal nerve cells, and HIV viruses only infect certain types of white blood cells in humans. The illustration below shows the infection process for an virus type called paramyxovirus that affects and causes a number of diseases in humans and other animals: