Do antibiotics work against viruses?
No, an antibiotic does nothing against a virus: for a cold, it does not speed up recovery and causes 2.6 times more side effects (Kenealy and Arroll, 2013). It targets parts that only bacteria have, such as their wall. Follow a cold and strep throat day by day.
Relative levels, a simplified diagram for a healthy adult. Each curve has its own scale. Dashed curves are estimates.
Do antibiotics work against viruses?
No, an antibiotic does nothing against a virus. It targets parts that only bacteria have, such as their cell wall. A virus has none: it gets into our cells and uses their machinery. Pick a germ, then give an antibiotic.
Sources: strep is found in 1 in 10 adults and 3 in 10 children with a sore throat (CDC). After one dose of amoxicillin, strep was undetectable the next day in 91% of children (Schwartz, Pediatr Infect Dis J, 2015). Antibiotic for a cold: no benefit, more side effects (Cochrane, 2013). Sizes: strep 0.5 to 1 µm (VetBact), influenza virus 85 to 120 nm (BioNumbers), rhinovirus about 30 nm (ViralZone). The days in the animation are simplified.
Keyboard: B bacterium, V virus, A antibiotic, Space pause, L slow motion, R restart.
Frequently asked questions about viruses, bacteria and antibiotics
What is the difference between a virus and bacteria?
A bacterium is a living cell that feeds and divides on its own, while a virus can only multiply inside our cells. A virus is also far smaller: the cold virus, the rhinovirus, is about 25 times smaller than a strep bacterium (30 nanometers versus 0.5 to 1 micrometer). That is why an antibiotic, which attacks the wall or the machinery that only bacteria have, finds nothing to target in a virus.
Viral or bacterial sore throat: how can you tell?
Only a rapid throat test can tell whether a sore throat comes from strep, the bacterium that calls for an antibiotic. Strep is the cause in only about 1 in 10 adults and 3 in 10 children with a sore throat (CDC). A cough, a runny nose or a hoarse voice point more toward a virus. The test is done at the doctor's office or, in some countries, at a pharmacy, with a result in a few minutes.
Green or yellow mucus: do you need an antibiotic?
Green or yellow mucus does not mean you need an antibiotic: its color comes from an enzyme in white blood cells, which show up whether the infection comes from a virus or from bacteria. In a large European study of adults with a cough, those who coughed up yellow or green mucus did not recover better with an antibiotic (Butler, Eur Respir J, 2011). What matters is how long it lasts: a cough that lasts more than 3 weeks, or symptoms that suddenly get worse, call for medical advice (NHS).
Can you stop an antibiotic as soon as you feel better?
It is best to follow the length of treatment set by the doctor, without shortening or extending it on your own, even if you feel better after a day or two. Against strep throat, amoxicillin works fast: the bacterium was no longer detected the next morning in 91% of children (Schwartz, 2015), but the recommended length in France is still 6 days (HAS, 2025). Leftover pills should go back to a pharmacy or a drug take-back site, rather than being kept or given to someone else.
How long does it take the gut to recover from an antibiotic?
After an antibiotic, gut bacteria get back close to normal in about 6 weeks, but some species can be missing for months. In 12 healthy men who had taken 3 strong antibiotics for 4 days, 9 species were still absent 6 months later in most of them (Palleja, Nature Microbiology, 2018). In the meantime, bacteria that resist antibiotics had taken up more room: one more reason to take an antibiotic only when it is useful.
True or false?
Antibiotics help you get over a cold faster.
False. A cold is caused by a virus, and antibiotics don't work on viruses. In adults, they cause 2.6 times more side effects, with no faster recovery (Kenealy and Arroll, 2013).
Strep throat can clear up without antibiotics.
True. Without antibiotics, 8 in 10 people have no more pain after a week. Treatment shortens the illness a little and reduces complications (Spinks, 2021).
On antibiotics, you stay contagious for several days.
False. For strep throat, the bacteria could no longer be detected by the morning after the first dose of amoxicillin in 91% of children (Schwartz, 2015).
This page as text
The same information as the charts, to read without animation: for each chart, the values at the main moments, then the notes with their level of evidence.
A cold, day by day
A cold comes from a virus, most often a rhinovirus. It gets into the cells of the nose and forces them to make more viruses. Our defenses are what clear it, in one to two weeks.
| Days since the first symptoms | Virus in the nose (dotted) | Runny nose, scratchy throat (dotted) | Cough (dotted) | Fast defenses (interferons, white blood cells) (dotted) | Antibodies (dotted) | What happens |
|---|---|---|---|---|---|---|
| −2 d | 2 | 0 | 0 | 0 | 0 | 2 days before the first symptoms |
| Day 0 | 70 | 40 | 5 | 55 | 0 | the first symptoms |
| +2 d | 85 | 95 | 44 | 89 | 2 | Day 2 after the first symptoms |
| +7 d | 18 | 25 | 40 | 26 | 61 | Day 7 after the first symptoms |
| +14 d | 2 | 3 | 12 | 8 | 100 | Day 14 after the first symptoms |
Key points
- The virus settles in (Before the symptoms, Shown in humans)
It takes about 2 days between contact with the virus and the first symptoms. Source: Lessler et al., « Incubation periods of acute respiratory viral infections: a systematic review », Lancet Infect Dis, 2009 - The peak, around day 2 (Days 0 to 3, Shown in humans)
The virus and the discomfort peak around day 2, then come down. Source: Gwaltney et al., « Symptom severity patterns in experimental common colds and their usefulness in timing onset of illness in natural colds », Clin Infect Dis, 2003 - Thick mucus, sometimes green (Days 1 to 7, Shown in humans)
The color comes from white blood cells that came to fight, not from bacteria. It is not a sign that you need an antibiotic. Source: Butler et al., « Antibiotic prescribing for discoloured sputum in acute cough/lower respiratory tract infection », Eur Respir J, 2011 - The cough lingers (From day 3, Shown in humans)
You feel better in 1 to 2 weeks, but the cough can last longer. After 3 weeks, it is best to see a doctor. Source: NHS, « Common cold » - An antibiotic changes nothing (Throughout, Shown in humans)
It does not help you get over a cold. In adults, it causes 2.6 times more side effects, mostly diarrhea. Source: Kenealy and Arroll, « Antibiotics for the common cold and acute purulent rhinitis », Cochrane, 2013
Strep throat, with or without an antibiotic
Most sore throats come from a virus. A small share comes from a bacterium, strep. In that case, an antibiotic stops it within a day, and the pain goes away a little faster.
| Days since the first symptoms | Strep in the throat, without an antibiotic (dotted) | Strep in the throat, with an antibiotic (dotted) | Sore throat, without an antibiotic (dotted) | Sore throat, with an antibiotic (dotted) | What happens |
|---|---|---|---|---|---|
| −2 d | 10 | 10 | 0 | 0 | 2 days before the first symptoms |
| Day 0 | 80 | 80 | 60 | 60 | the first symptoms |
| +2 d | 99 | 6 | 90 | 75 | Day 2 after the first symptoms |
| +7 d | 60 | 0 | 15 | 8 | Day 7 after the first symptoms |
| +14 d | 25 | 0 | 0 | 0 | Day 14 after the first symptoms |
Key points
- Virus or bacteria? A test tells (Days 0 to 1, Shown in humans)
Strep causes about 1 in 10 sore throats in adults and 3 in 10 in children. A cough, a runny nose or a hoarse voice point more toward a virus. A rapid throat test gives the answer in a few minutes. Source: CDC, « Strep throat: all you need to know » - The antibiotic stops the bacteria (Days 1 to 2, Shown in humans)
The morning after a first dose of amoxicillin, strep could no longer be detected in 91% of children. Source: Schwartz et al., « A reappraisal of the minimum duration of antibiotic treatment before approval of return to school for children with streptococcal pharyngitis », Pediatr Infect Dis J, 2015 - A few fewer days of pain (Days 1 to 7, Shown in humans)
At 3 days, 30% fewer people still have pain with an antibiotic. Without an antibiotic, 8 in 10 people no longer have pain after a week. Source: Spinks et al., « Antibiotics for treatment of sore throat in children and adults », Cochrane, 2021 - Why treat it anyway (From day 1, Shown in humans)
The antibiotic lowers the risk of complications such as rheumatic fever, now rare in wealthy countries, and you quickly stop being contagious. In France, the standard is amoxicillin for 6 days. Source: HAS (French National Authority for Health), « Choix et durées d’antibiothérapies : angine aiguë de l’adulte », updated 2025
What an antibiotic does to gut bacteria
An antibiotic does not pick and choose: it also hits the helpful bacteria in the gut. The ones that resist it take over for a while. Getting back to normal takes weeks.
| Days since the first symptoms | Variety of gut bacteria (dotted) | Bacteria that resist the antibiotic (dotted) | Risk of diarrhea (dotted) | What happens |
|---|---|---|---|---|
| −2 d | 100 | 10 | 0 | 2 days before the first symptoms |
| Day 0 | 100 | 10 | 0 | the first symptoms |
| +2 d | 88 | 22 | 30 | Day 2 after the first symptoms |
| +7 d | 45 | 75 | 50 | Day 7 after the first symptoms |
| +14 d | 70 | 45 | 0 | Day 14 after the first symptoms |
Key points
- The good bacteria too (Days 1 to 7, Shown in humans)
During treatment, the variety of gut bacteria drops. Some less helpful bacteria take advantage. Source: Palleja et al., « Recovery of gut microbiota of healthy adults following antibiotic exposure », Nat Microbiol, 2018 - Diarrhea is possible (Days 1 to 10, Shown in humans)
It affects 5 to 35% of people, depending on the antibiotic and the person. Source: McFarland, « Antibiotic-associated diarrhea: epidemiology, trends and treatment », Future Microbiol, 2008 - Resistant bacteria take over (From day 2, Shown in humans)
Bacteria that carry resistance genes are favored during and after treatment. Source: Palleja et al., « Recovery of gut microbiota of healthy adults following antibiotic exposure », Nat Microbiol, 2018 - A slow recovery (From day 7, Shown in humans)
After 4 days of 3 strong antibiotics, the gut was close to normal within 6 weeks, but 9 species were still missing at 6 months in most of the volunteers. Source: Palleja et al., « Recovery of gut microbiota of healthy adults following antibiotic exposure », Nat Microbiol, 2018
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