Respiratory Infections
Colds and flu — and the antibiotics that do nothing for either
What it is
Acute respiratory infections are the most common infectious illnesses in the world. The common cold is caused by more than two hundred viruses, most often rhinovirus; influenza is a distinct and far more serious viral illness. Both are viral. Neither responds to antibiotics.
Why it matters
Two corrections here have real consequences. First, antibiotics do nothing for a cold or for flu — and prescribing them anyway is one of the largest avoidable drivers of antibiotic resistance in the world, alongside side effects for the individual who gains nothing. Green mucus does not change this, and the belief that it does is one of the most reliably wrong things people know. Second, influenza is not a bad cold: it is a serious illness that kills, and the widespread habit of calling any cold 'the flu' has flattened a distinction that matters clinically.
What BioSignal knows about treating this
What works for Respiratory Infections
BioSignal’s clinical summary, most important first.
- Rest, fluids, time. Most colds resolve in 7–10 days; a cough can persist for three weeks
- Symptomatic relief — analgesia, decongestants
- Influenza vaccination — the single most effective preventive measure
- Antiviral treatment for influenza in high-risk patients, started early
- Hand hygiene
- Antibiotics do NOTHING for viral respiratory infection and cause harm at population scale
Signal Records relevant to this condition
Interventions and contributing factors — some of these records describe a cause rather than a cure. The rating shown is BioSignal’s confidence in that Signal Record, not a claim about how well it treats this condition. Open any of them for the full evidence.
- Vitamin CHigh confidence
Does not prevent colds — one of the best-tested negatives in nutrition. Shortens them slightly, but only if you were already taking it. Megadoses are mostly excreted: absorption saturates around 200 mg/day. Genuinely useful for iron absorption, and genuinely essential — scurvy is still real.
- ZincHigh confidence
Strong evidence where there is deficiency or a specific indication. No benefit in replete people — including for testosterone — and excess causes copper deficiency.
- Vitamin DHigh confidence
Effective for deficiency and for bone health in at-risk groups. For broad disease prevention in adults who are already replete, the largest trials are null — and confidence in that null is high. Routine testing of healthy adults is not supported.
- ProbioticsLimited evidence
Specific strains help specific conditions. 'Probiotics for gut health' in healthy adults is not supported — fibre has the better evidence.
New to this? Read these first
How this usually unfolds
- Recognize risk factors
- Get diagnosed
- Track key biomarkers
- Lifestyle first
- Evidence-based treatment
- Long-term monitoring
Who is at risk
- Close contact and crowded indoor environments — the main driver of winter seasonality
- Children in the household
- Age — very young and very old
- Chronic lung, heart or metabolic disease
- Immunosuppression (including from autoimmune disease treatment)
- Smoking
- Vitamin D deficiency (a modest contributor)
How it's diagnosed
Colds are diagnosed clinically and need no testing. Influenza can be confirmed by rapid antigen or PCR testing where it changes management — for example, whether to give antiviral treatment to someone at high risk. The clinically useful distinction is the pattern: a cold builds gradually and stays mostly above the neck; influenza tends to arrive abruptly with fever, muscle aches and prostration.
Key biomarkers
Lifestyle
Explore this condition across BioSignal
Related Foundations
Related Signal Records
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Frequently asked questions
Does vitamin C prevent colds?
No. Taken regularly, it does not reduce how often you catch a cold — that question has been tested in dozens of randomised trials over more than fifty years and the answer is settled. It does modestly shorten a cold, but only if you were already taking it before you got ill, which is not how anyone actually uses it. There is one real exception: in people under extreme physical stress — marathon runners, skiers, soldiers in subarctic conditions — cold incidence was roughly halved. If you are not one of those, the sachet is not going to help. See our Vitamin C record.
Why do I keep getting sick?
Usually the answer is exposure, not a defective immune system — and that is a more useful answer than it sounds. Young children are the single biggest factor: households with them see far more infections, because children are efficient vectors. Crowded indoor environments, commuting, poor sleep and smoking all raise your exposure or lower your defences. Adults catch a few colds a year and that is normal. If you are getting frequent, SEVERE, or unusual infections — infections needing intravenous antibiotics, or in unusual sites — that is a different question and is worth investigating properly, because genuine immunodeficiency does exist. But it is rare, and it is not what a run of winter colds means.
My mucus is green. Do I need antibiotics?
No — and this is one of the most reliably wrong things people believe about their own bodies. Green or yellow mucus does not mean bacterial infection. The colour comes from enzymes released by your own immune cells as they do their job, and it is an entirely normal part of the course of a viral cold. Antibiotics do nothing to a virus. They will not shorten your illness by an hour, they carry side effects, and they contribute to antibiotic resistance, which is a genuine global problem built one unnecessary prescription at a time.
Is the flu just a bad cold?
No, and the casual use of 'flu' for any cold has blurred a distinction that genuinely matters. Influenza is a distinct illness with a distinct virus, it arrives abruptly with fever and severe muscle aches, and it kills people every year — particularly older adults and those with chronic conditions. A cold makes you feel unwell; influenza tends to put you in bed. Vaccination is the single most effective thing you can do about it, its effectiveness varies year to year, and it remains worthwhile.
Does zinc help a cold?
Possibly, modestly, and with caveats. Zinc lozenges started early may shorten cold duration somewhat, and the evidence is genuinely better than for vitamin C on that specific question — though it is heterogeneous, the formulations differ, and many people find the taste unpleasant enough to stop. One clear warning: intranasal zinc products should be avoided entirely, having been associated with lasting loss of smell. If you want the most honest summary: zinc is worth a try for duration, it is not prevention, and it is not a treatment.
Evidence summary
Acute respiratory infections are overwhelmingly viral; the common cold is most often caused by rhinovirus, and antibiotics have no effect on viral infection while contributing materially to antimicrobial resistance. Sputum colour does not predict bacterial infection and should not guide antibiotic prescribing. Influenza is clinically distinct from the common cold and carries substantial morbidity and mortality; vaccination is the most effective preventive intervention, with effectiveness varying by season. Regular vitamin C supplementation does not reduce cold incidence in the general population but modestly reduces duration, and roughly halves incidence in people under extreme physical stress. Zinc lozenges, taken early, may modestly reduce cold duration, with heterogeneous evidence; intranasal zinc has been associated with anosmia and should be avoided. Vitamin D supplementation shows a modest reduction in acute respiratory infection, concentrated in those who are deficient. Short sleep duration is associated with increased susceptibility to experimentally induced rhinovirus infection.
References & sources
- Cochrane reviews of vitamin C for the common cold
- Cochrane reviews of antibiotics for upper respiratory tract infection
- Meta-analysis of vitamin D supplementation and acute respiratory tract infection
Educational information — not medical advice
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