In short

Viral infections are illnesses caused by tiny organisms called viruses that invade your body's cells to make copies of themselves. From the common cold to more serious conditions like the flu or COVID-19, these infections affect millions of people worldwide and can range from mild to life-threatening.

What is a Viral Infection?

A viral infection happens when a virus gets into your body and starts multiplying inside your cells. Viruses are very small germs—much smaller than bacteria—that you can only see under a microscope. Unlike bacteria, viruses cannot survive or reproduce on their own. They must invade living cells to make more copies of themselves.

There are thousands of different viruses, but only a small number can infect humans. Some viral infections are mild and go away on their own, like the common cold. Others can be more serious and may cause life-threatening or long-lasting (chronic) illnesses.

How Viruses Work in Your Body

A virus is made of genetic material—either DNA (deoxyribonucleic acid) or RNA (ribonucleic acid)—wrapped in a protective coating called a capsid. You can think of it like an envelope carrying instructions. Our cells, on the other hand, are like complete factories that contain instructions and all the equipment needed to carry them out.

Because viruses don't have cells of their own, they lack the machinery needed to make copies of themselves. To reproduce, they must break into your cells and hijack your cell's equipment to do it. The virus attaches to a host cell, enters it, and releases its genetic material inside. This genetic material takes control of the cell and forces it to make new viruses.

The infected cell usually dies because the virus prevents it from performing its normal functions. When the infected cell dies, it releases new viruses that go on to infect other cells. This process of viruses replicating is what makes you sick with a viral infection. Sometimes the process can kill, damage, or change infected cells, which can make you feel ill. The symptoms can range from mild to very severe.

Viruses usually infect only one particular type of cell. For example, common cold viruses only infect cells of the upper respiratory tract, and hepatitis viruses affect cells in the liver.

Types of Viral Infections

Viral infections can affect different parts of your body. You might hear viruses described by which part of your body they infect, how they spread, or what symptoms they cause. Here are the main types:

Respiratory Infections

Respiratory infections affect your nose, throat, airways, and lungs. Many respiratory viruses can cause bronchitis (inflammation of the airways), sinusitis (sinus infection), ear infections, or pneumonia (lung infection). Examples include:

Common cold (usually caused by rhinovirus)
The flu (influenza)
COVID-19
Respiratory syncytial virus (RSV)
Human metapneumovirus (hMPV)
Parainfluenza

Upper respiratory infections are probably the most common viral infections. Respiratory infections are more likely to cause severe symptoms in infants, older adults, and people with lung or heart problems.

Digestive System Infections

Viral infections in your digestive system can affect your stomach and intestines (called the GI tract) or your liver. These include:

Norovirus, rotavirus, and astrovirus can cause gastroenteritis, sometimes called "stomach flu"
Hepatitis viruses cause liver disease and often last a long time

Viral Hemorrhagic Fevers

Hemorrhagic fevers affect how your blood clots and can weaken your blood vessels, causing potentially life-threatening bleeding. Examples include Ebola, Hantavirus pulmonary syndrome, severe dengue, and yellow fever.

Sexually Transmitted Viruses

Sexually transmitted diseases are mainly spread by sexual contact, including oral, anal, and vaginal sex. Examples include:

Human immunodeficiency virus (HIV)
Human papilloma virus (HPV)/genital warts
Genital herpes (HSV)
Hepatitis B

Exanthematous Viral Infections

Exanthematous viral infections cause rashes that can appear as bumps or blisters on your skin or spots of blood under your skin. They can also cause respiratory or other symptoms.

Neurological Infections

Some viruses infect the brain, causing encephalitis (brain inflammation). Others infect the layers of tissue that cover the brain and spinal cord. Examples include the rabies virus and the West Nile virus.

How Viruses Spread

Viruses can spread in different ways, depending on the type. Common ways you can catch a virus include:

Through droplets and particles that are breathed out by someone who has the infection. You might breathe in the droplets or particles, or they could land on your mouth, nose, or eyes
By touching surfaces or objects that have the virus on them and then touching your mouth, nose, or eyes
From the pregnant parent to the baby during pregnancy
Through contaminated food or water
By being bitten by an infected insect or animal
Through sexual contact (usually vaginal, anal, and oral sex) with someone who has the infection

Once inside your body, the virus uses your own cells to make copies of itself. This process can trigger your immune system, causing the symptoms of an infection.

Common Symptoms

The signs and symptoms of a viral infection depend on what virus you have and how it affects your body. Because there are so many viruses, symptoms can vary widely. Common signs include:

Fever or chills
Cough, sore throat, or runny nose
Headache or body aches
Fatigue (tiredness) or weakness
Nausea, vomiting, or diarrhea (in stomach viruses)
Skin rashes (in viruses like chickenpox or measles)

Some symptoms may start mildly and worsen over time, while others go away within a few days. Other times, your immune system may be able to fight off the virus and you may not have any symptoms at all.

How Viral Infections Differ from Bacterial Infections

Viruses and bacteria can cause similar symptoms, like fever, cough, and rashes. The only way to know what kind of infection you have is to have a healthcare provider assess you. If you have symptoms that last more than a few days or that concern you, see your provider.

How Viral Infections Are Diagnosed

To diagnose a viral infection, your healthcare provider may:

Ask about your symptoms and recent exposure to illness
Do a physical exam
Take a sample of blood, mucus, saliva, or stool
Use rapid tests, especially for viruses like COVID-19, flu, or RSV
Order blood tests or imaging if symptoms are severe or unusual

In many cases, the doctor can diagnose a viral infection based on your symptoms and the time of year, such as during flu season. Your doctor may need a sample to see what sort of infection you have, as it's important to know if your infection is caused by bacteria or viruses because the treatments differ.

Treatment Options

Unlike bacterial infections, antibiotics do not work on viruses. Antibiotics only treat infections caused by bacteria and are ineffective against viral infections. Most viral infections get better with rest and supportive care.

Home Care and Symptom Management

For most viral infections, treatments can only help with symptoms while you wait for your immune system to fight off the virus. The good news is that viral infections usually aren't serious. Most will go away in a few days without medical treatment. Here are ways to care for yourself at home:

Get plenty of rest if you feel tired. Sleep helps your body fight the virus and get better
Drink plenty of fluids to avoid dehydration. If you have kidney, heart, or liver disease and have to limit fluids, talk with your doctor first
Use over-the-counter medicine like acetaminophen (Tylenol) or ibuprofen (Advil, Motrin) to reduce fever or pain. Do not give aspirin to anyone younger than 18, as it has been linked to Reye syndrome, a serious illness
Eat healthy foods and nutritious foods to support your immune system
Stay home from work, school, and other public places while you have a fever

Antiviral Medications

There are antiviral medicines to treat some viral infections. Some serious viral infections may be treated with antiviral medications, such as oseltamivir (Tamiflu) for flu, or antiviral drugs for COVID-19, herpes, hepatitis, or HIV.

These medicines can help shorten the illness or prevent complications, but they must be taken early in the infection. Antiviral drugs, if given within 2 days after the onset of symptoms, will prevent the infection and replication of viruses. However, antiviral drugs are not suitable in every case. They can be toxic to human cells and may cause side effects such as fever, chills, weakness, and muscle pain.

How to Prevent Viral Infections

Vaccines can help prevent you from getting many viral diseases. The best treatment for many viral infections is prevention. You may be able to prevent some viral infections by:

Proper hand washing with soap and water, and using an alcohol-based hand sanitizer when needed
Avoiding touching your face, especially your eyes and nose, without washing your hands first
Paying attention to food safety and eating fresh fruits and vegetables
Cleaning surfaces that may be infected with germs
Avoiding contact with wild animals
Preventing insect bites by using insect repellent when you go outdoors
Practicing safe sex (using a condom every time you have anal, vaginal, or oral sex)
Avoiding close contact with people who are sick
Getting enough sleep—most people need an average of seven hours of sleep a night to keep their immune system strong
Staying up-to-date with all recommended vaccines

When You're Sick

If you develop a viral infection, taking precautions when you are sick can help stop the spread of respiratory viruses to others. Stay home and away from others (including people you live with who are not sick) if you have respiratory virus symptoms.

You can go back to your normal activities when, for at least 24 hours, both are true: your symptoms are getting better overall, and you have not had a fever (and are not using fever-reducing medication). When you go back to normal activities, take added precautions over the next 5 days, such as practicing good hygiene, wearing a well-fitted mask, physical distancing, or testing when you will be around other people.

Recovery Time and Outlook

A viral infection usually lasts only a week or two. Viral infection may last for only 1-2 weeks. The length of time a virus lasts can vary greatly depending on the specific virus and the individual infected.

Most viral infections go away in a few days without medical treatment. When your symptoms are getting better overall and you have not had a fever for at least 24 hours, you are typically less contagious, but it still takes more time for your body to fully get rid of the virus. During this time, you may still be able to spread the virus to others.

You should contact your doctor or nurse advice line if you seem to be getting much sicker, you have a new or higher fever, you have a severe headache or stiff neck, you have blood in your stools, you have new belly pain or your pain gets worse, you have a new rash, or you are confused or disoriented.

Some viruses can cause chronic infections that last for years, even decades. In many people, chronic infections are quite mild, but in others they can result in serious complications. Almost any virus can lead to sepsis, a life-threatening emergency that happens when your body's response to an infection damages vital organs.