by Kathy Rusniak

Created on: February 10, 2009

Having a cold or the flu is no fun. In either case, you feel sick, have a hard time concentrating at school or work, and are potentially spreading your 'germs' to anyone in your vicinity. Both the common cold and the flu are caused by viruses, those nasty little bits of inert nucleic acids with protein coats that are nonetheless readily transmissible from person to person, especially throughout the winter season. But this is where the similarity in these two illnesses ends. The cold and flu are caused by different virus species, have different modes of infection, and have decidedly contrasting levels of virulence.

The common cold is caused by viruses from several families, genera, multiple species within genera, and also many subtypes within species. For example, the most familiar virus that can cause a cold is the rhinovirus, which belongs to the family Picornaviridae. The rhinovirus has over one hundred subtypes, or forms of the same virus but with different antigens presented on the viruses outer protein coat. Other cold-related groups include corona virus, adenovirus, human parainfluenza, metapneumovirus and human respiratory syncitial virus. Alternately, the flu is caused by viruses from the family Orthomyxoviridae, with just three genera, InfluenzavirusA, InfluenzavirusB and InfluenzavirusC. These groups each have just one species as a member, Influenza A, Influenza B, and Influenza C. Each species may have multiple subtypes.

Cold viruses have proteins on their coat that have an affinity for cells in the nasal area of the host. They attach to I-CAM receptors on cells in the nasal cavity and pharynx where they then infect the cells and multiply. Infected cells are not killed during the cold virus's reproductive cycle. In contrast, Influenza viruses have a proteins on their outer coats called hemoglutinnin A, which bind to sialic acid protein receptors on cells and allow them to attach to cells of the nasal passages, throat and lungs of the host. Then an ion channel is created in the membrane of the host cell so the virus's genetic material (RNA) can be introduced. Once new viral particles have been produced, a protein called neuraminidase cleaves portions of the new viral protein coats to release the viruses and spread them to other host body cells. Host cells die once the new viral particles are released.

Infection with a cold virus results in generally uncomfortable symptoms such as a sore throat, nasal discharge, and mild aches or sensitivity throughout

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