Write a short note on viruses.

(N/A) $\rightarrow$ Viruses are non-cellular organisms characterized by having an inert crystalline structure outside the living cell.
$\rightarrow$ They are not truly 'living' and thus did not find a place in the classification system.
$\rightarrow$ Once they infect a host cell,they take over the host's machinery to replicate themselves,eventually killing the host.
$\rightarrow$ The term 'virus',meaning venom or poisonous fluid,was given by Pasteur.
$\rightarrow$ $D. I. Ivanowsky (1892)$ recognized certain microbes as the causal organisms of the mosaic disease of tobacco,which were smaller than bacteria as they passed through bacteria-proof filters.
$\rightarrow$ $M. W. Beijerinck (1898)$ demonstrated that the extract of infected tobacco plants could cause infection in healthy plants and called the fluid 'Contagium vivum fluidum' (infectious living fluid).
$\rightarrow$ $W. M. Stanley (1935)$ showed that viruses could be crystallized,and these crystals consist largely of proteins.
$\rightarrow$ Viruses are obligate parasites.
$\rightarrow$ Structure: They contain genetic material,which could be either $RNA$ or $DNA$,but never both.
$\rightarrow$ $A$ virus is a nucleoprotein,and the genetic material is infectious.
$\rightarrow$ Plant viruses usually have single-stranded $RNA$,while animal viruses have single or double-stranded $RNA$ or double-stranded $DNA$.
$\rightarrow$ Bacteriophages (viruses that infect bacteria) are usually double-stranded $DNA$ viruses.
$\rightarrow$ The protein coat,called the capsid,is made of small subunits called capsomeres,which protect the nucleic acid and are arranged in helical or polyhedral geometric forms.
$\rightarrow$ Viruses cause diseases like mumps,smallpox,herpes,influenza,and $AIDS$ in humans.