Common cold

• 1. Introduction

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  The common cold is the most frequent infectious disease in humans, and the average adult catches two to three of them every year. A child may catch six to eight. School children can reach up to twelve in a single year. These infections have existed throughout human history, and yet there is still no vaccine and no cure. Well over 200 virus strains can cause one. So why has medicine, which has conquered far deadlier diseases, never beaten this one? Why do the symptoms come from your own body rather than the virus? And what does a cold actually cost a country that spends billions trying to relieve it? The answers begin with a mistake almost everyone makes about how a cold starts.

• 2. The Cold Weather Myth

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  A common misconception is that one can catch a cold merely through prolonged exposure to cold weather. The name itself encourages the error. The word "cold" came into use in the 16th century, because its symptoms resembled those of exposure to cold weather. Colds are viral infections, not a product of temperature alone. Yet the prevalence of many of these viruses is genuinely seasonal, occurring more frequently during cold weather. The reason for that seasonality has not been conclusively determined. Possible explanations include cold temperature-induced changes in the respiratory system and a decreased immune response. Low humidity may also raise viral transmission rates, perhaps because dry air lets small viral droplets disperse farther and stay airborne longer. The apparent seasonality may also be social. People spend more time indoors near infected people in winter, and children gather at school. Normal exposure to cold temperatures does not increase the risk of infection. Severe exposure that significantly lowers body temperature, producing hypothermia, may be a different matter. The point is controversial, but the majority of evidence suggests it may increase susceptibility to infection.

• 3. An Army Of Viruses

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  More than 200 viral types are associated with colds, which is the central reason the disease has never been defeated. The most common are rhinoviruses, responsible for up to 40% of cases, and they alone have over 100 known serotypes. Other frequent culprits include coronaviruses, adenoviruses, enteroviruses, parainfluenza, and RSV. Frequently more than one virus is present at the same time. The viral cause of some colds, between 20% and 30%, is simply unknown. Rhinovirus is typically acquired by direct contact and binds to humans via ICAM-1 receptors and the CDHR3 receptor through mechanisms that are still unknown. It does not generally damage the nasal epithelium, the thin protective layer of cells on the surface of the nose. RSV behaves differently. It is contracted by direct contact and airborne droplets, replicates in the nose and throat, then spreads to the lower respiratory tract, and it does cause epithelium damage. Human parainfluenza virus inflames the nose, throat, and bronchi. In young children, when it affects the trachea, it can produce the symptoms of croup because their airways are so small. This variety, and the rapid mutation of these viruses, is why creating a broadly effective vaccine is highly improbable.

• 4. Your Body's Overreaction

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  The symptoms of a cold are believed to be primarily related to the immune response to the virus rather than to tissue destruction by the viruses themselves. When rhinovirus binds to its receptors, it triggers the release of inflammatory mediators, and those mediators produce the symptoms you feel. The mechanism is virus-specific. A cold usually begins with fatigue, a feeling of being chilled, sneezing, and a headache, followed a couple of days later by a runny nose and cough. A sore throat is present in about 40% of cases, a cough in about 50%, and muscle aches in about 50%. In adults, a fever is generally absent, but it is common in infants and young children. The color of the mucus, which may range from clear to yellow to green, does not reveal whether the infection is viral or bacterial. Symptoms may begin within sixteen hours of exposure and typically peak two to four days after onset. They usually resolve in seven to ten days, though some can last up to three weeks. The average duration of cough is eighteen days, and some people develop a post-viral cough that lingers after the infection is gone. In children, the cough lasts more than ten days in 35 to 40% of cases and continues for more than 25 days in 10%.

• 5. How It Spreads

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  The common cold is transmitted by airborne droplets, by contact with nasal secretions, or by contaminated objects. Breathing, talking, or coughing releases small droplets carrying the virus, which can infect someone who inhales them or touches their eyes or nose. The viruses survive on surfaces for prolonged periods, over 18 hours for rhinoviruses, and hands carry them to the eyes or nose. Daycare and schools are powerful transmitters because many children with little immunity and poor hygiene are packed close together, and the infections then travel home to the rest of the family. People sitting close to one another appear to be at greater risk, but there is no evidence that recirculated air during commercial flight spreads the disease. Transmission from animals is considered highly unlikely. One outbreak at a British scientific base on Adelaide Island, which struck after seventeen weeks of isolation, was thought to have come from a contaminated object or an asymptomatic human carrier, rather than from the husky dogs also present at the base. Herd immunity from previous exposure limits viral spread, which is why younger populations, with less of it, suffer greater rates of respiratory infection. Insufficient sleep and malnutrition have both been linked to greater risk after rhinovirus exposure, believed to be because of their effects on immune function. A mother with a cold cannot transmit it through breast milk, and by the time she shows symptoms her baby has already been exposed, so breastfeeding is recommended to continue.

• 6. Why Nothing Cures It

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  There is no cure for the common cold, and treatment instead aims to relieve symptoms. Much of the benefit of symptomatic treatment is attributed to the placebo effect, and no medications or herbal remedies have been conclusively shown to shorten the infection. Getting plenty of rest, drinking fluids for hydration, and gargling with warm salt water are reasonable conservative measures. Pain and fever medications such as ibuprofen and acetaminophen may help, though it is unclear whether acetaminophen helps with symptoms. Cough medicines are not recommended for children because of a lack of evidence and the potential for harm. In 2009, Canada restricted over-the-counter cough and cold medication in children six years and under. Antibiotics have no effect against viral infections, yet they are still frequently prescribed, which significantly increases the development of antibiotic resistance. Zinc supplements taken as pills may shorten colds by up to 33% and reduce symptom severity if supplementation begins within 24 hours of onset, although some zinc remedies applied inside the nose have caused loss of the sense of smell. Vitamin C does not affect how often people catch colds, but may reduce duration if taken regularly. Echinacea and garlic have no firm evidence of meaningful benefit. The only reliable protection against cold-laden aerosols comes from airborne precautions such as respirators, ventilation, HEPA, and high MERV filters, since droplet precautions alone cannot reliably stop inhalation.

• 7. The Hunt For A Treatment

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  In the United Kingdom, the Common Cold Unit was set up by the Medical Research Council in 1946, and it was there that the rhinovirus was discovered in 1956. While the cause of the common cold was identified in the 1950s, the disease itself is ancient. Its symptoms and treatment are described in the Egyptian Ebers papyrus, the oldest existing medical text, written before the 16th century BCE. In the 1970s, the Common Cold Unit showed that treatment with interferon during the incubation phase of rhinovirus infection offered some protection, but no practical treatment could be developed from it. The unit closed in 1989, two years after it completed research on zinc gluconate lozenges for the prevention and treatment of rhinovirus colds, the only successful treatment in the history of the unit. The search continues elsewhere. As of 2009, no antiviral had been both found effective and licensed for use against the common cold. Trials of the antiviral pleconaril showed promise against picornaviruses, and trials of BTA-798 were also underway. The oral form of pleconaril had safety issues, so an aerosol form is being studied. The genomes of all known human rhinovirus strains have now been sequenced.

• 8. The Price Of A Cold

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  In the United States, the common cold leads to 75 to 100 million physician visits annually, at a conservative cost estimate of $7.7 billion per year. Americans spend $2.9 billion on over-the-counter drugs and another $400 million on prescription medicines for symptom relief. More than one-third of people who saw a doctor received an antibiotic prescription, carrying implications for antibiotic resistance. The disease drains time as much as money. An estimated 22 to 189 million school days are missed each year because of a cold. Parents missed 126 million workdays to stay home and care for their children, and employees who themselves had a cold missed another 150 million workdays. Together these losses push the total economic impact of cold-related work loss past $20 billion per year, which accounts for 40% of time lost from work in the United States. Severe complications are rare and tend to strike the very old, the very young, or the immunosuppressed. Secondary bacterial infections can follow, with sinusitis estimated in 8% of cases and ear infection in 30%. With a basic reproduction number of 2 to 3, each infected person typically passes the cold to two or three others, which is how the most common human disease keeps circling the globe.