Colds and flu

Colds and flu are both respiratory infections caused by viruses, and they are routinely confused with each other, in both directions. People describe a heavy cold as flu, and occasionally dismiss flu as a bad cold and carry on until it flattens them.

The difference is worth knowing because it changes what is reasonable to expect, what helps, and when it is worth asking for help.

What causes each

A cold can be caused by any of a large number of viruses. Rhinoviruses are the commonest, with more than a hundred varieties of those alone, which is why colds recur throughout life — immunity to one is no protection against the next. Coronaviruses, respiratory syncytial virus, adenoviruses and parainfluenza viruses also produce colds.

Flu is caused specifically by influenza viruses, chiefly influenza A and B. These change from year to year, which is why the vaccine is reformulated annually and why having had flu once confers little protection later.

One piece of biology that circulates in a garbled form: a virus does not rewrite a cell's DNA to become a copy of itself, and viruses are not cells. A virus is a set of genetic instructions in a protein coat. It gets inside a cell and uses that cell's machinery to manufacture more copies of itself, which then infect further cells. There is nothing to be done about that process directly, which is why most treatment is aimed at the symptoms rather than the virus.

Telling them apart

The useful distinction is in the onset and in which symptoms dominate.

A cold builds gradually over a day or two. It sits in the nose and throat: sneezing, a blocked or running nose, a sore throat, a cough, mild aches. A temperature is possible but usually slight. Most people keep going, feeling wretched. It is worst around day three and largely over within a week to ten days, though a cough can linger for two or three weeks.

Flu arrives abruptly, often within hours, and the whole body is involved: a high temperature, aching muscles, headache, exhaustion, and the particular feeling of being unable to get out of bed rather than choosing not to. A dry cough and sore throat come with it. The acute phase is a few days to a week; the tiredness can run on for a fortnight or more.

Three honest caveats. The distinction is not reliable in individual cases, particularly in older people and small children. Testing is not usually done outside hospital. And since 2020 the list has lengthened — COVID-19 produces overlapping symptoms and cannot be separated from either on symptoms alone, which is what tests are for when it matters.

What actually helps

Neither has a cure, and both resolve on their own in most people. What is left is making the week tolerable:

• Rest and fluids. Unglamorous and the main thing.
• Paracetamol or ibuprofen for fever, headache and aching. Worth checking that a combined cold remedy does not already contain paracetamol before adding more.
• Honey for a cough, which has better evidence behind it than most cough medicines and is recommended in UK guidance as a first option. Not for babies under one year.
• Saline drops or spray for a blocked nose, and decongestants for a few days only, since longer use rebounds.
• Steam or menthol vapour for a blocked nose, which produces a genuine sensation of clearer breathing — the mechanism and the practical limits are under using essential oils safely.
• Zinc lozenges are the one supplement with a plausible case for shortening a cold, and the evidence remains inconsistent. Taste and nausea put many people off.

Two popular measures that do less than their reputation. Vitamin C does not prevent colds in people who are not deficient and not under extreme physical stress; taken regularly it may shorten one marginally, and started after symptoms begin it does nothing. And antibiotics do not work on viruses at all — they have no effect on a cold or on flu, and taking them for one contributes to resistance while risking side effects for no return.

Antiviral medicines for flu do exist and are a different matter. They are for people at higher risk of complications, they need starting within about forty-eight hours of symptoms beginning, and they are available through a GP or NHS 111 during flu season. For anyone in a clinical risk group, that time limit is the reason to ring early rather than wait and see.

The vaccine

Annual flu vaccination is the single most effective measure available, and it was absent from older writing on this subject entirely.

It is offered free on the NHS to: people aged 65 and over; anyone pregnant; adults and children with certain long-term conditions, including asthma, diabetes, heart, kidney, liver and neurological conditions, and anyone immunosuppressed; children from toddler age through much of primary and secondary school; carers; care home residents; and health and social care workers.

Children are given it as a nasal spray rather than an injection, which has been routine in the UK for well over a decade — it is not a new development. The vaccine changes each year to match the strains in circulation, which is why last year's does not count. It is possible to be vaccinated and still catch a different strain; the vaccine reduces the chance of infection and, more importantly, the severity if it happens.

A COVID booster is offered each autumn to a narrower list of groups, and the two are frequently given at the same appointment.

Not catching one, and not passing it on

Both spread mainly through the air — in droplets and finer particles breathed, coughed and sneezed out — and secondarily through hands and surfaces. Older accounts that describe transmission as a matter of kissing and holding hands understate the airborne route considerably, which is the main one for flu.

So: covering coughs and sneezes, disposing of tissues, washing hands, ventilating rooms, and staying away from work and from vulnerable people while feverish. There is nothing exotic available, and the measures that sound too simple are the ones that work.

On the phrase boosting immunity, which attaches itself to this subject every winter: a normally functioning immune system cannot be boosted, and products promising to do it are not describing anything real. What genuinely reduces the number of infections a person gets is sleep, not smoking, vaccination and hand hygiene, which is a duller list and an accurate one.

The remedies, assessed

A great deal is sold for colds and flu. Most of it does nothing, which is a waste rather than a danger. Two items in common circulation are different, and both were recommended in older writing on this subject.

Hydrogen peroxide placed in the ears should not be done. The claim behind it is that flu enters the body through the ear canals, and that is simply not how it works — the ear canal is a dead end closed off by the eardrum, with no route to the airways. Putting hydrogen peroxide into an ear causes pain, irritation and dizziness, and where the eardrum is perforated it can reach the middle ear and do real damage.

Colloidal silver should not be taken. It is marketed as a natural antibiotic and antiviral and is neither. Taken over time it causes argyria, a permanent blue-grey discolouration of the skin that does not reverse, and it interferes with the absorption of several medicines including some antibiotics and thyroid treatment. Medicines regulators have warned against it repeatedly.

The rest are ineffective rather than harmful. Echinacea has been tested a great deal and does not consistently prevent colds or shorten them. Elderberry rests on a few small, weak trials. Oscillococcinum is a homeopathic preparation of duck liver and heart diluted past the point where any of it remains, and reviews find no evidence that it does anything. Ginseng is unproven and does interact with a number of prescribed medicines, which is a reason for caution rather than a benefit.

None of these is a reason for alarm. The cost of them is the week spent on something that does not work, and occasionally a delay in getting flu antivirals to someone who would have benefited from them.

When to get help

Most colds and most flu need nobody. These do:

• Difficulty breathing, breathlessness at rest, or chest pain.
• Coughing up blood.
• Confusion, drowsiness, or a stiff neck with a severe headache.
• Getting better and then getting worse again, especially with a returning fever, which can signal a bacterial infection on top.
• A cough lasting more than three weeks.
• A baby under three months with a temperature, or any child who is floppy, not feeding, or has a rash that does not fade under pressure — these are urgent.
• Anyone in a clinical risk group, pregnant, elderly or immunosuppressed who develops flu symptoms, early enough for antivirals to be an option.

NHS 111 is the route when it is unclear, and 999 for severe breathing difficulty or someone who cannot be roused.