Explain the term Viruses

Viruses was demonstrated by Iwananowski  in 1892, bacteria were considered the lowest forms of life. Viruses  are  called  as  'obligate  intracellular parasites',  since they are unable  to carry out any of the typical life functions until they are inside a host cell. Once inside the host cell, they thrive and direct the host cell to produce more viruses. As long as the virus is outside the host cell, it is known as  'vision'.

The viruses are minute when compared to bacteria. Except for a  few, viruses like the cow pox virus, used in vaccination against small pox is  0.3  pm,  whereas, the smallest type, like the foot and mouth disease virus  is about 0.01 pm. Note, they are so  small  that they cannot  be  seen under  an  electron microscope.

The viruses consist of a protein layer, capsid,  surrounding nucleic acid comprising either RNA  or DNA. The  important characteristic of viruses is that  they  are host- specific. Most viruses infect only one species, either animal or plant or else only very closely related species. The mammalian viruses do not affect any prant e.g. the polio virus infects humans and monkey and does not affect other animals, whereas, the tobacco mosaic virus, which attacks plants, does not affect humans. Viruses are killed  in  a  few minutes under pasteurization temperature  i.e.  62°C for 30 minutes. They are affected by  general disinfectants like phenols, formaldehyde, halogens  and  cresols.  To  a certain extent, soaps and detergents inactivate them and  UV  light destroys  all  viruses.  They  are not  affected  by antibiotics  unlike bacteria.

Viruses are known to cause illness although they do not grow on  foods or produce toxins  in  foods. Food items merely act as a vehicle for their transfer. They  are the intestinal or enteric type and  are food borne. They spread from the hands of human carriers and from water to foods. The presence of viruses  in  foods, especially the shell fish grown in sewage polluted water, could be the significant route of illness in man.  Other  foods like  hits and vegetables contaminated by  faces and  salad preparations using contaminated vegetables have been implicated in several outbreaks of food borne diseases. The hepatitis A virus, which causes jaundice, spreads through Food