ConditionInfections
Viral Infections
A viral infection is an illness caused by a virus, a tiny germ that can only multiply inside living cells. Viruses cause most colds, sore throats, flu, stomach bugs, COVID-19 and many rashes, as well as long-term infections such as hepatitis B and C and HIV. Most short-term viral infections clear on their own within 1 to 2 weeks with rest and fluids. Antibiotics do not work against viruses, but antiviral medicines treat some, and vaccines prevent many.
Also called: Viral illness, viral disease, virus infection, viral syndrome
At a glance
- What it is
- Illness caused by a virus invading and multiplying in the body's cells.
- Common examples
- Colds, flu, COVID-19, RSV, viral gastroenteritis, chickenpox, mono, cold sores.
- Long-term examples
- Hepatitis B and C, HIV, herpes viruses (which stay dormant).
- Typical duration
- Most acute infections improve within 7 to 10 days; cough can linger 2 to 3 weeks.
- Treatment
- Rest, fluids, symptom relief; antivirals for flu, COVID-19, herpes, hepatitis and HIV.
- Prevention
- Vaccines, handwashing, staying home when sick.
When to get help
Call 911 or go to the ER
- Difficulty breathing, blue or gray lips, chest pain or confusion: call 911 or your local emergency number
- A stiff neck, severe headache and a rash that does not fade under pressure, with fever
- Signs of severe dehydration: no urine for 8 hours or more, very dry mouth, dizziness, a floppy or unresponsive child
- A baby under 3 months with a temperature of 38 °C (100.4 °F) or higher
See a doctor within a day or two
- Symptoms that improve, then come back worse after several days (possible bacterial complication)
- Fever above 39.4 °C (103 °F) or lasting more than 3 days
- Flu or COVID-19 symptoms in someone at high risk, such as over 65, pregnant, or with chronic disease or weak immunity (antivirals work best within 2 days)
- Yellow skin or eyes, dark urine
Book a routine appointment
- Symptoms lasting more than 10 days without improvement
- Possible exposure to HIV or hepatitis: testing and, for HIV, post-exposure prevention within 72 hours
Usually safe to manage at home
- A cold with runny nose, sore throat and mild cough, improving within a week
- A mild stomach bug with vomiting or diarrhea where you can keep fluids down
How viruses cause illness
Viruses are much smaller than bacteria. They contain genetic material (DNA or RNA) in a protein coat. To reproduce, a virus enters a cell and hijacks its machinery to make copies, often damaging the cell. Symptoms come from both cell damage and the immune response, such as fever, aches and inflammation.
Some viruses are cleared completely. Others, such as herpes viruses, stay dormant in the body for life and can reactivate. Some, such as HIV and hepatitis B, cause chronic infection.
How viruses spread
- Respiratory droplets and aerosols: colds, flu, COVID-19, RSV, measles.
- Touching contaminated surfaces and then the eyes, nose or mouth.
- Food and water: norovirus, hepatitis A.
- Blood and body fluids: HIV, hepatitis B and C.
- Sexual contact: HIV, herpes, HPV.
- Insect bites: dengue, West Nile, Zika.
- Mother to baby: during pregnancy, birth or breastfeeding for some viruses.
Common types of viral infections
| System | Examples | Typical symptoms |
|---|---|---|
| Respiratory | Common cold, flu, COVID-19, RSV | Runny nose, sore throat, cough, fever, aches |
| Digestive | Gastroenteritis (norovirus, rotavirus) | Vomiting, diarrhea, cramps |
| Liver | Viral hepatitis A, B, C | Fatigue, jaundice, dark urine |
| Skin and rashes | Chickenpox, measles, cold sores, warts | Rashes, blisters, sores |
| Systemic | Mononucleosis, dengue | Fever, fatigue, swollen glands |
| Chronic | HIV, hepatitis B and C | Often no symptoms for years |
| Nervous system | Viral meningitis, encephalitis | Headache, stiff neck, confusion |
Viral vs bacterial infection
It can be hard to tell them apart, and doctors sometimes need tests. General patterns:
| Viral | Bacterial | |
|---|---|---|
| Onset | Often with several symptoms (nose, throat, cough) | Often focused in one area |
| Duration | Improves within 7 to 10 days | May worsen or persist |
| Mucus color | Can be yellow or green normally | Color alone does not prove bacteria |
| Fever | Often moderate, early | Can be high or rise later |
| Antibiotics | Do not help | Often needed |
Clues to a bacterial infection or complication include symptoms lasting more than 10 days, worsening after initial improvement, high fever, severe one-sided ear or facial pain, or a very sore throat without cough.
Diagnosis
Most common viral infections are diagnosed from symptoms. Tests are used when they change treatment or public health action:
- rapid tests and PCR swabs for flu, COVID-19, RSV and strep (to rule out bacteria)
- blood tests for hepatitis, HIV, mono and dengue
- stool tests for some gut viruses
- swabs of blisters for herpes or chickenpox
Treatment
Self-care
- rest and fluids
- acetaminophen (paracetamol) or ibuprofen for fever and aches; follow the label dose, and do not give aspirin to children or teenagers
- saline sprays, honey for cough in adults and children over 1 year, and humidified air
- oral rehydration solution for vomiting and diarrhea
Antiviral medicines
- Influenza: oseltamivir and others, best within 48 hours, especially for high-risk people.
- COVID-19: nirmatrelvir-ritonavir and others for people at higher risk.
- Herpes viruses: acyclovir, valacyclovir for cold sores, genital herpes and shingles.
- Hepatitis B and C: long-term suppression for B; curative tablets for C.
- HIV: antiretroviral therapy controls the virus and prevents transmission.
Why antibiotics do not help
Antibiotics kill bacteria, not viruses. Using them for viral infections does not speed recovery, causes side effects and drives antibiotic resistance.
Prevention
- Vaccines: flu, COVID-19, MMR, chickenpox, shingles, hepatitis A and B, HPV, polio, rotavirus, RSV for eligible groups.
- Hand hygiene: wash with soap and water; alcohol gel helps for many viruses but not norovirus.
- Cover coughs and sneezes, and stay home when sick.
- Safer sex and not sharing needles to prevent blood-borne and sexually transmitted viruses.
- Mosquito protection when traveling to areas with mosquito-borne viruses.
Frequently asked questions
How long does a viral infection last?
Most common viral infections, like colds and stomach bugs, last 3 to 10 days, though a cough can linger 2 to 3 weeks. Flu and COVID-19 can cause fatigue for longer. Some viruses, like hepatitis B or HIV, can cause long-term infection.
How do I know if an infection is viral or bacterial?
Viral infections often affect several areas at once, such as nose, throat and chest, and improve within 7 to 10 days. Bacterial infections are more often focused in one area and may worsen. A doctor may use tests to tell them apart.
Do antibiotics work for viral infections?
No. Antibiotics only kill bacteria. They do not shorten viral illnesses and can cause side effects and antibiotic resistance. Some viral infections can be treated with specific antiviral medicines.
What are the most common viral infections?
The most common are the common cold, flu, COVID-19, RSV and viral gastroenteritis (stomach bugs). Others include cold sores, chickenpox, mono, viral conjunctivitis and warts.
Sources
This guide was written from current patient and clinical guidance, including:
- CDC: Antibiotic prescribing and use, viruses or bacteria
- CDC: Vaccines and immunizations
- World Health Organization: Infectious diseases fact sheets
- MedlinePlus: Viral infections
- NIH National Institute of Allergy and Infectious Diseases: Viral diseases research