How is the Common Cold Spread?

How Rhinovirus Infects Humans

Most colds are caused by rhinoviruses, which are spread by coughing and sneezing and infect the nasal airway.

The common cold is a viral upper respiratory infection characterized by coughing, sore throat, sneezing, runny nose, nasal congestion, and fatigue. The symptoms can be caused by any of more than 200 viruses, but nearly half are attributed to rhinovirus.

What Is Rhinovirus?

Rhinoviruses, of which there are three known genetic subtypes consisting of roughly 100 serotypes, belong to the family Picornaviridae. This is the same family as the virus that causes polio. Rhinoviruses are stable, meaning that they can exist outside the body for hours. However, the virus is sensitive to temperature, so it primarily infects the upper respiratory rather than the lower respiratory tract. Because of the broad range of types of rhinoviruses, multiple infections over the course of a year and lifetime are quite common and easily spread.

How a Cold is Spread

Viral respiratory infections, such as those that cause the common cold, are spread by aerosol. The virus is sneezed or coughed out into the air, and the droplets are inhaled by another person. Containing the spray from a cough or sneeze can prevent the spread of a cold. Viruses can also spread in the mucus secretions from the respiratory tract, and spread can be limited by frequent hand washing.

Simply inhaling or touching the virus to susceptible surfaces of the body is not enough for infection. The virus has to enter the cells of the respiratory tract, the mucosa. The body has defenses that prevent infection, including producing the mucus and phlegm that induces congestion, runny noses, and coughing. Pathogens have evolved mechanisms to overcome these defenses. In particular, rhinovirus overcomes host defenses by presenting an overwhelming amount of virus to mucosal surfaces. Single droplets from a sneeze can contain 100 million rhinovirus particles.

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How Rhinovirus Causes Cold Symptoms

When rhinoviruses infect epithelial cells, specialized immune cells recognize the presence of the pathogen. The immune response is characterized not only by the production of antibodies against the virus but also the production of mucus and slight inflammation, a process known so well by cold sufferers. This excess mucus causes congested nasal passages, leading to a runny nose, itchy eyes, and sneezing. It also causes congested airways, leading to coughing and sore throat.

Once a virus infects host cells, it generally hijacks the cellular machinery to replicate, destroying the host cells and increasing the amount of virus in the body. This, in effect, kills and damages the tissue targeted by infection. However, rhinoviruses are self-limiting (the infection and symptoms subside on their own within a few days to weeks) and do not usually result in long-term damage to the tissue.

Recent research has found that rhinovirus manipulates some human genes to create an exaggerated immune response, exacerbating the effect its presence has on the infected individual. In particular, the virus up-regulates pro-inflammatory cytokines, a necessary evil of the immune response. Inflammation can cause fatigue and aches, but recruits immune cells to kill and clear the virus.

Common Cold Symptoms

The symptoms of the acute upper respiratory infection caused by rhinoviruses include coughing, sneezing, sore throat, stuffy and runny nose, and itchy eyes. The symptoms are all the result of the virus invading the cells of the upper respiratory system or the immune response to its presence. The first response to the presence of the pathogen is the secretion of mucus in the nose and respiratory tract (sneezing, stuffy, and runny nose). The mucus is an attempt by the body to prevent the virus from entering the cells of the mucus membranes. The immune response then causes slight inflammation, resulting in itchy and irritated membranes (sore throat, itchy eyes, coughing).

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Children often develop a fever with the infection, but it is not common in adults. Headaches, muscle aches, and fatigue may also develop as the body defends itself and uses up resources to rid itself of the pathogen. Rhinoviruses have a short incubation period of roughly eight to 10 hours but may linger, according to the Centers for Disease Control and Prevention (CDC), for up to two weeks.

References:

Flint, S. J. et al. Principles of Virology: Molecule Biology, Pathogenesis, and Control. ASM Press, Washington DC, 2000.

Hunt. Richard. Virology Ch 10, part 2: Rhinoviruses. University of South Carolina School of Medicine, Microbiology and Immunology Online.

Scientific Blogging News Staff. Cold Virus ‘Manipulates’ Genes. scientificblogging.com Published October 24, 2008

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