ConditionInfections
Viral Infection
A viral infection is an illness caused by a virus multiplying inside the body's cells. Common examples are colds, flu, COVID-19, stomach bugs, chickenpox and cold sores; more serious ones include hepatitis, HIV and viral meningitis. Typical symptoms are fever, tiredness, aches, sore throat, cough, runny nose or diarrhea, and most short-term viral infections clear on their own within 7 to 10 days. Antibiotics do not work against viruses; rest, fluids, symptom relief and, for some viruses, antiviral medicines and vaccines are the main tools.
Also called: Viral illness, virus infection, viral syndrome
At a glance
- What it is
- Illness caused by a virus replicating inside the body's cells.
- Common examples
- Colds, flu, COVID-19, RSV, stomach bugs, mono, chickenpox, cold sores.
- Typical symptoms
- Fever, tiredness, aches, sore throat, runny nose, cough, diarrhea, rash.
- Duration
- Most respiratory viruses last 7 to 10 days; a cough can linger for weeks.
- Treatment
- Rest, fluids, symptom relief; antivirals for flu, COVID-19, herpes, HIV and hepatitis.
- Antibiotics
- Do not work against viruses.
When to get help
Call 911 or go to the ER
- Difficulty breathing, blue or gray lips, or chest pain
- Confusion, extreme drowsiness, a seizure, or a stiff neck with severe headache
- A rash that does not fade when pressed, with fever
- Signs of severe dehydration: no urine for 8 hours, dizziness, fainting, a very sleepy child
See a doctor within a day or two
- Any fever of 38 °C (100.4 °F) or higher in a baby under 3 months
- Fever above 39.4 °C (103 °F), or fever lasting more than 3 days
- Symptoms that improve then return worse (possible bacterial complication)
- Flu or COVID-19 symptoms in someone at high risk (over 65, pregnant, chronic illness, weak immune system), ideally within 48 hours for antivirals
- Yellow skin or eyes, or dark urine
Book a routine appointment
- Fatigue or cough lasting more than 3 to 4 weeks after an infection
- Frequent infections
Usually safe to manage at home
- Cold symptoms, mild fever and aches improving over a few days
- A mild stomach bug while you can keep fluids down
How viruses cause illness
Viruses are tiny germs that cannot reproduce on their own. They enter cells, take over the cell's machinery to make copies of themselves, and spread to new cells. Symptoms come from both the cell damage and the immune system's response, such as fever, inflammation and fatigue.
After infection, the immune system usually clears the virus and builds memory against it. Some viruses, such as herpesviruses and HIV, can remain in the body for life.
Common viral infections
| Body area | Examples |
|---|---|
| Nose, throat and airways | Common cold, flu, COVID-19, RSV, viral sore throat, bronchiolitis |
| Gut | Norovirus and rotavirus (viral gastroenteritis) |
| Skin | Chickenpox, shingles, cold sores, warts, hand, foot and mouth disease, measles |
| Liver | Hepatitis A, B, C, D, E |
| Whole body | Mononucleosis, HIV, dengue |
| Nervous system | Viral meningitis, encephalitis |
Symptoms
General symptoms of many viral infections:
- fever and chills
- tiredness and weakness
- muscle and joint aches
- headache
- loss of appetite
Symptoms specific to the area affected:
- respiratory: runny or blocked nose, sneezing, sore throat, cough, hoarseness
- gut: nausea, vomiting, diarrhea, cramps
- skin: rashes, blisters
- liver: yellow skin and eyes, dark urine
Viral vs bacterial infection
| Feature | Viral | Bacterial |
|---|---|---|
| Common examples | Colds, flu, most sore throats, most bronchitis | Strep throat, UTIs, many pneumonias, cellulitis |
| Onset | Often gradual, with several symptoms | Often more localized |
| Course | Peaks in 2 to 3 days, improves in 7 to 10 | May worsen without treatment |
| Mucus color | Yellow or green mucus is common and does not mean bacteria | Not reliable for diagnosis |
| Antibiotics | Do not help | Often needed |
Bacterial infections sometimes follow viral ones, such as sinusitis, ear infections or pneumonia after a cold or flu. Warning signs include symptoms lasting more than 10 days, a second worsening after initial improvement, high fever, or localized severe pain.
Diagnosis
Most common viral infections are diagnosed from symptoms. Tests are used when they change treatment:
- rapid tests or PCR for flu, COVID-19 and RSV
- strep test to rule out bacterial sore throat
- blood tests for mono, hepatitis or HIV
- stool tests in outbreaks
Treatment
Self-care
- rest
- fluids: water, broths, oral rehydration solutions
- acetaminophen (paracetamol) or ibuprofen at the label dose for fever and aches; never aspirin for children or teenagers
- saline nasal sprays, honey for cough in people over 1 year, throat lozenges for older children and adults
- humidified air
Antiviral medicines
Some viruses have specific antivirals:
- flu: oseltamivir, baloxavir, best started within 48 hours
- COVID-19: nirmatrelvir-ritonavir, remdesivir, for higher-risk people
- herpes viruses: acyclovir, valacyclovir for cold sores, genital herpes, shingles
- HIV: antiretroviral therapy
- hepatitis B and C: long-term or curative antiviral regimens
Why not antibiotics?
Antibiotics kill bacteria, not viruses. Taking them for viral infections does not speed recovery and causes side effects and antibiotic resistance.
How long viral infections last
- colds: 7 to 10 days; cough up to 3 weeks
- flu: fever 3 to 5 days, tiredness up to 2 weeks
- stomach bugs: 1 to 3 days
- mono: fatigue can last weeks to months
Some people have lingering fatigue or cough after viral infections, and a minority develop long-term conditions such as long COVID.
Prevention
- vaccines: flu, COVID-19, RSV for older adults and infants, measles, chickenpox, shingles, hepatitis A and B, HPV, rotavirus, polio
- handwashing with soap and water
- covering coughs and sneezes
- staying home when sick
- safe sex and not sharing needles for blood-borne viruses
Frequently asked questions
What are the symptoms of a viral infection?
Common symptoms include fever, tiredness, aches, headache, sore throat, runny nose, cough, and sometimes vomiting, diarrhea or a rash. Symptoms depend on which virus and which part of the body is affected.
How do I know if an infection is viral or bacterial?
Viral infections often cause several symptoms at once, such as a runny nose, sore throat and cough, and improve within 7 to 10 days. Bacterial infections are more often localized and may worsen without treatment. A doctor may use tests to tell them apart.
How long does a viral infection last?
Most common viral infections, like colds, last 7 to 10 days, though a cough can linger for weeks. Flu typically lasts about a week with tiredness for longer. Some viruses, such as mono, can cause fatigue for weeks to months.
Do antibiotics help viral infections?
No. Antibiotics do not work against viruses. They are only useful if a bacterial complication develops. Antiviral medicines exist for specific viruses such as flu, COVID-19, herpes, HIV and hepatitis.
Sources
This guide was written from current patient and clinical guidance, including:
- CDC: Antibiotics: Are You Using Them Right?
- CDC: Flu Symptoms and Complications
- MedlinePlus: Viral Infections
- World Health Organization: Influenza (seasonal) fact sheet
- NHS: Viral infections and antibiotics
- National Institute of Allergy and Infectious Diseases: Viral diseases