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Biology

The viruses behind colds and flu

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We’re taking a detour into biology for today’s graphic, looking at the colds that many of us are suffering from at this time of year. It’s doubly topical considering the coronavirus outbreak in China at the time of writing. This graphic highlights the viruses that cause colds and flu and their different characteristics.

Most of us won’t make it through the whole of winter without developing a cold at some point. On average, adults have between two and five colds every year. Children have an average of between seven and ten. Over 200 different viral types are associated with colds, and these can be a little more succinctly divided into several key families.

The major cold-causing viruses in humans are rhinoviruses. This virus isn’t named after the horny African/Asian mammal, but from the Greek ‘rhinos’, meaning ‘of the nose’. This name relates to the fact that rhinoviruses replicate best at the temperatures found in the nose (33–35˚C). For the linguistically curious, the word ‘rhinoceros’ is also of Greek derivation, meaning ‘nose-horned’.

There are three species of rhinovirus that affect humans: A, B and C. Within these there are a large number of different serotypes. Serotypes are viruses that differ in their surface proteins. Over 150 different serotypes of rhinovirus are known, and overall they account for 30-50% of all colds.

Coronaviruses are the next most common virus family behind colds, with seven known species that affect humans. They’ve been behind some high profile cases, including the SARS outbreak several years ago, and the current coronavirus outbreak in China. The symptoms of colds caused by coronaviruses tend to be more severe than those caused by rhinoviruses, and they can also cause pneumonia. Their name comes from the Latin for crown, ‘corona’, for their bulbous, crown-like surface projections.

Another virus type that can cause more serious symptoms is the influenza virus. Infections with this virus are usually referred to as flu, with symptoms including fever. There are 12 known serotypes of influenza A virus, which is the most common in humans and causes yearly flu outbreaks around the world.

Unlike most other cold viruses, you can get a vaccine against influenza viruses. This is far from practical – as a number of viruses cause colds and flu, and they mutate rapidly, it’s challenging to develop a vaccination that works, and impossible to produce a vaccination that would protect against all cold-causing viruses. However, due to the more serious symptoms and potential for deaths in the elderly and vulnerable, yearly flu vaccines are given. In particular, they’re provided to at-risk groups.

Twice a year, the World Health Organisation directs the production of the flu vaccination based on predictions of the strains of the virus most likely to be circulating. While it does not guarantee that those vaccinated won’t contract flu, most years it provides relatively good protection. The vaccination is only as good as the predictions; if the dominant strains differ from those predicted, the vaccine may be less effective.

Other treatment options for colds are few and far between. As colds and flu are caused by viral infections, antibiotics, which treat bacterial infections, are not effective. There is minimal evidence for the effectiveness of other touted treatments such as vitamin C, garlic and honey. There is some evidence that zinc acetate lozenges reduce the duration of cold symptoms by up to a third if they’re taken within 24 hours of the onset of symptoms.

We don’t even know everything there is to know about the viruses that cause colds and flu. In addition to the viruses we’ve already encountered, several others can cause colds. Respiratory syncytial virus, parainfluenza virus, adenoviruses, enteroviruses, and metapneumovirus are all further types. In addition to these, in 20-30% of colds, the virus causing it can’t be identified. In cases it may be a known virus, but others may be as-yet-undiscovered cold-causing viruses.

In most cases, if you have a cold, you’re infected by just one of the viruses we’ve noted here. In about 5% of cases, though, people with colds are infected with more than one virus type simultaneously. You’re not always likely to know which type of virus you’re infected with, since the treatment options (or lack of) are the same regardless, so it’s not particularly useful to identify. Still, there are some things you can do to relieve the symptoms – and chemistry can help out there, as this graphic in C&EN shows.

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