Flu, Cold, and COVID: How to Tell Them Apart and What to Do
In autumn and winter, most of us fall ill at least once. Runny nose, fever, sore throat, body aches — familiar symptoms. But a "cold", the flu, and COVID are very different conditions with different risks and different management approaches. Knowing which one you have matters: it determines whether home treatment is safe, whether you need a doctor, and whether antibiotics are needed. This article covers the key differences and what to do in each case.
Flu vs cold: the key differences
The common cold is a collective term for over 200 viruses, including rhinoviruses, adenoviruses, coronaviruses (other than SARS-CoV-2), parainfluenza, and others. Influenza is a distinct infection caused by influenza viruses type A, B, or C.
How to tell if it's flu rather than a cold:
- Onset. A cold comes on gradually: a slight sore throat today, a runny nose tomorrow. Flu hits suddenly — people often remember the exact hour they started feeling unwell.
- Fever. Colds often cause a moderate temperature (37–38°C) or none at all. Flu typically causes a high fever (38.5–40°C) that rises sharply.
- General condition. With a cold: "I'm sick, but I can still function." With flu: severe weakness, aching joints and muscles, inability to get out of bed. Headache with flu is nearly universal — often very intense.
- Catarrhal symptoms. Runny nose, cough, sore throat are typical of a cold. With flu, these may be minimal or absent at first — the dominant features are systemic (fever, weakness, aching).
- Cough. With flu — dry, painful, may persist 2–3 weeks.
Reliably telling flu from a cold without a test is not possible. Rapid flu tests are available at pharmacies with moderate accuracy. For a reliable result — PCR.
COVID: how it presents now
Since the emergence of vaccines and newer variants, COVID-19 runs a significantly milder course in most vaccinated or previously infected people compared to 2020–2021. Current variant symptoms often resemble a cold or flu.
Features more typical of COVID:
- loss or change in smell and/or taste (especially characteristic of early variants; less common with Omicron);
- marked breathlessness;
- severe fatigue out of proportion to other symptoms;
- diarrhoea or nausea at the start of illness.
Reliably distinguishing COVID from flu and cold without a test is not possible. COVID antigen rapid tests are widely available. A positive result means isolation and a doctor's consultation — especially with risk factors (elderly, immunosuppression, chronic conditions).
When to see a doctor and when home treatment is fine
Home treatment is appropriate for a cold or mild flu if:
- you are an adult without serious underlying conditions;
- the fever is manageable and responds to paracetamol or ibuprofen;
- symptoms are gradually improving;
- there are no signs of complications (see below).
See a doctor if:
- fever above 39°C that won't come down;
- fever lasting more than 5 days;
- shortness of breath, chest pain;
- confusion, severe headache, neck stiffness;
- cough with purulent or blood-streaked sputum;
- infant under 3 months with a temperature above 38°C;
- elderly (65+), pregnant women, people with chronic conditions (diabetes, heart disease, immunosuppression).
Call emergency services immediately for:
- severe breathlessness at rest;
- cyanosis (blue lips or fingertips);
- loss of consciousness;
- seizures.
Antibiotics for colds and flu: when they're needed, when they're not
This is one of the most important questions — and one of the most widespread myths in medicine.
Colds and flu are caused by viruses. Antibiotics do not act on viruses. Taking an antibiotic for a viral infection does not speed recovery, reduce fever, or relieve symptoms. What it does do is promote antibiotic resistance and disrupt the gut microbiome.
When antibiotics are genuinely needed during a respiratory illness:
- Bacterial complications — pneumonia (confirmed on chest X-ray), purulent otitis media, bacterial sinusitis (symptoms lasting more than 10–14 days without improvement or with deterioration), streptococcal tonsillitis (confirmed by rapid test).
- The decision is always made by a doctor.
Yellowish or greenish phlegm is not an indication for antibiotics. It is a normal immune response, not evidence of a bacterial infection.
Long COVID: persistent symptoms
In some people, symptoms persist for more than 12 weeks after the acute illness — this is called post-COVID syndrome, or Long COVID. It is estimated to affect 10–30% of those who have had COVID, though the rate has declined with Omicron and widespread vaccination.
The most common Long COVID symptoms:
- marked fatigue that doesn't improve with rest;
- breathlessness on exertion;
- "brain fog" — reduced concentration and memory;
- persistent loss of smell or taste;
- muscle and joint pain;
- rapid heartbeat;
- sleep disturbances;
- anxiety and depressive symptoms.
The mechanisms behind Long COVID are not fully understood. Proposed explanations include chronic inflammation, autoimmune reactions, autonomic nervous system dysfunction, and reactivation of herpesviruses. Management is symptomatic and conducted under medical supervision.
If you're unsure how serious your symptoms are and whether to see a doctor — try describing them to the assistant. It will help assess urgency and suggest which specialist to see.
Frequently asked questions
Should you lower a fever during flu?
Fever up to 38.5°C is a protective response that helps fight the virus. There's no need to bring it down if it's well-tolerated. Above 38.5°C or with poor tolerance — paracetamol or ibuprofen. Aspirin must never be given to children under 18 during viral illnesses (risk of Reye's syndrome).
Do antiviral medications work for flu?
Oseltamivir (Tamiflu) and zanamivir are medications with proven effectiveness against influenza A and B. They shorten illness by 1–2 days when taken within the first 48 hours. They are prescribed primarily to high-risk groups and in severe cases. Most popular "antivirals" available without a prescription lack solid evidence.
Can you catch COVID twice?
Yes. Post-infection immunity is partial and temporary. Reinfection risk is particularly high with new variants — Omicron, for instance, differs significantly from earlier strains. Vaccination reduces the risk of severe illness even when it doesn't fully prevent infection.
How long is a person contagious with a cold?
With most colds, the contagious period runs from 1–2 days before symptoms appear through 5–7 days after illness begins. Flu is similar. With COVID, contagiousness can persist for up to 10 days or longer in immunocompromised patients. Two negative tests are the standard for ending COVID isolation.
Do antiviral nasal sprays protect against infection?
Nasal sprays containing carrageenan or similar substances may modestly reduce infection risk by creating a barrier on the nasal lining. The evidence base is moderate. Masks, hand hygiene, and ventilation remain more reliable protective measures.
Symptomatica is an informational reference service. Not a medical service; does not diagnose or prescribe treatment. For any symptoms, please consult a doctor.