Viruses
Coronaviruses are a group of viruses known for causing the common cold. They have a halo or crown-like (corona) appearance when viewed under an electron microscope.
The common cold is a viral infection of the upper respiratory tract. The most commonly implicated virus is a rhinovirus (30%–80%), a type of picornavirus with 99 known serotypes.[27][28] Other commonly implicated viruses include human coronavirus (≈15%),[29][30] influenza viruses (10%–15%),[31] adenoviruses (5%),[31] human respiratory syncytial virus, enteroviruses other than rhinoviruses, human parainfluenza viruses, and metapneumovirus.[32] Frequently more than one virus is present.[33] In total over 200 different viral types are associated with colds
Transmission
The common cold virus is typically transmitted via airborne droplets (aerosols), direct contact with infected nasal secretions, or fomites (contaminated objects).[7][34] Which of these routes is of primary importance has not been determined; however, hand-to-hand and hand-to-surface-to-hand contact seems of more importance than transmission via aerosols.[35] The viruses may survive for prolonged periods in the environment (over 18 hours for rhinoviruses) and can be picked up by people’s hands and subsequently carried to their eyes or nose where infection occurs.[34] Transmission is common in daycare and at school due to the proximity of many children with little immunity and frequently poor hygiene.[36] These infections are then brought home to other members of the family.[36] There is no evidence that recirculated air during commercial flight is a method of transmission.[34] People sitting in close proximity appear to be at greater risk of infection.[35]
Rhinovirus-caused colds are most infectious during the first three days of symptoms; they are much less infectious afterwards.
Weather
The traditional theory is that a cold can be “caught” by prolonged exposure to cold weather such as rain or winter conditions, which is how the disease got its name.[38] Some of the viruses that cause the common colds are seasonal, occurring more frequently during cold or wet weather.[39] The reason for the seasonality has not been conclusively determined.[40] Possible explanations may include cold temperature-induced changes in the respiratory system,[41] decreased immune response,[42] and low humidity causing an increase in viral transmission rates, perhaps due to dry air allowing small viral droplets to disperse farther and stay in the air longer.[43]
The apparent seasonality may also be due to social factors, such as people spending more time indoors, near infected people,[41] and specifically children at school.[36][40] There is some controversy over the role of low body temperature as a risk factor for the common cold; the majority of the evidence suggests that it may result in greater susceptibility to infection
Herd immunity, generated from previous exposure to cold viruses, plays an important role in limiting viral spread, as seen with younger populations that have greater rates of respiratory infections.[44] Poor immune function is a risk factor for disease.[44][45] Insufficient sleep and malnutrition have been associated with a greater risk of developing infection following rhinovirus exposure; this is believed to be due to their effects on immune function.[46][47] Breast feeding decreases the risk of acute otitis media and lower respiratory tract infections among other diseases,[48] and it is recommended that breast feeding be continued when an infant has a cold.[49] In the developed world breast feeding may not be protective against the common cold in and of itself.