Viruses are tiny, ranging in size
from about 20 to 400 nanometers
in diameter. Billions can fit on the head of a pin. Some are rod shaped; others are round and 20 sided; and yet others have fanciful forms, with multisided “heads” and cylindrical “tails.”
Viruses are responsible for a wide range of diseases, including the common cold, measles, chicken pox, genital herpes, and influenza.
Viruses are simply packets of nucleic acid, either DNA
, surrounded by a protein
shell and sometimes fatty materials called lipids
. Outside a living cell, a virus is a dormant particle, lacking the raw materials for reproduction. Only when it enters a host
cell does it go into action, hijacking the cell’s metabolic
machinery to produce copies of itself that may burst out of infected cells or simply bud off a cell membrane
. This lack of self-sufficiency means that viruses cannot be cultured in artificial media for scientific research or vaccine
development; they can only be grown in living cells, fertilized eggs
, tissue cultures
, or bacteria.
Viruses are responsible for a wide range of diseases, including the common cold, measles
, chicken pox
, genital herpes, and influenza
. Many of the emerging infectious diseases, such as AIDS
, are caused by viruses.
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