Bacterial and viral infections: How doctors tell the difference

When germs like bacteria or viruses get into your body, you can get something called an infection. Sometimes this means a runny nose and a sore throat, like with a cold. Other times, it can be more serious, like pneumonia.

But not all infections are the same. Some are due to bacteria, and others come from viruses.

Why does this matter? Knowing which type of infection you have changes how doctors treat it. Viral infections usually get better on their own with rest and care at home. But bacterial infections sometimes need medicines called antibiotics.

What causes bacterial infections?

Bacterial infections happen because of tiny living things called bacteria. Your body is home to bacteria that live inside it all the time. Most are harmless, and some even help you stay healthy. But certain kinds of bacteria cause problems. Some bacteria can grow too much or get into places they shouldn’t, like a cut or your lungs.¹

You can get a bacterial infection in a few ways:

• Touching someone who’s sick, like through hugging or kissing
• Breathing in germs from a sneeze or cough
• Eating food that isn’t cooked enough, or drinking water that isn’t clean
• Having a cut or scrape that bacteria can get into

Some examples of bacterial infections include:

• Strep throat
• Urinary tract infections (UTIs)
• Bacterial pneumonia (a lung infection)
• Whooping cough

How long bacteria live on surfaces depends on the type of bacteria and the environment. Some bacteria can live as long as a few months on hard surfaces.

What causes viral infections?

Viral infections come from viruses. These are even smaller than bacteria. Viruses can’t live on their own. They need to get inside your healthy cells to grow, and that can make you sick.²

You might get a viral infection by:

• Breathing in germs from someone who coughs or sneezes nearby
• Touching surfaces with germs on them, then touching your face
• Eating or drinking things with germs in them
• Touching bodily fluids like blood or spit
• Getting bitten by an insect, like a mosquito or tick

Some common viral infections are:

• The common cold
• The flu
• COVID-19
• Chickenpox
• Stomach flu

Viruses can spread easily. For example, the flu virus causes millions of infections each year in the United States. In the 2023 to 2024 flu season, about 40 million people got sick with the flu.³

Viruses don’t live as long as bacteria on surfaces. Usually, they can survive up to 48 hours on hard surfaces. But some viruses can stay around longer in the air.

Bacterial and viral infection symptoms

Bacterial and viral infections can make you feel very similar. That’s why it can be hard to tell them apart. Different germs can cause different problems, depending on where they are in your body.

Most bacterial and viral infections have these symptoms in common:

• Fever
• Feeling tired or worn out
• Cough
• Runny nose
• Achy muscles

If you look more closely, you might notice some differences. The only way to know for sure is for a doctor to give a diagnosis.

Possible signs of a bacterial infection:

• Symptoms that don’t go away (like a high fever)
• You feel pain in one spot, like your ear, sinuses, or when you pee
• Thick green or yellow mucus or pus
• A cut looking red, puffy and hot to the touch

Possible signs of a viral infection:

• Symptoms that start quickly but get better in about a week or two
• Clear runny nose, but feeling stuffy
• Achy muscles all over, but no pain in just one area

Treatment

It helps to know what’s causing your infection. If you’re dealing with a bacterial infection, doctors may give you antibiotics. The kind of antibiotics depends on what bacteria’s making you sick. But you should only take antibiotics if your doctor prescribes them to you.

Antibiotics will not help viruses. Most of the time, rest, fluids, and over-the-counter medicines are enough to help you recover from a viral infection. But some serious viral infections might need antiviral medicine.⁴

If you take antibiotics when you don’t need them, bacteria can start to get stronger and harder to treat. This is called antibiotic resistance. In the United States, about 2.8 million people each year get infections that are hard to treat because of antibiotic resistance. Over 35,000 people die each year from these infections.⁵

When should someone see a healthcare professional?

It’s not always easy to know if your infection is due to a bacteria or a virus. Sometimes the signs look the same. If your symptoms get worse or you’re unsure if you need medical treatment, it’s a good idea to ask for help.

Some signs you should call your doctor include:

• Fever over 103°F that doesn’t get better with medicine⁶
• Trouble breathing
• Chest pain
• A really bad headache
• Stiff neck
• Vomiting or diarrhea that won’t stop
• Feeling so tired you can’t get up from bed
• Seizures
• Your symptoms get better, but then they come back worse
• Your symptoms last more than 2 weeks

Infections can get serious quickly for some people, like if you have cancer or HIV. If you have a weak immune system, call your doctor as soon as you feel sick.

Takeaway

Bacterial and viral infections can look very similar. But they need different treatments. Symptoms depend on what part of your body is affected and what germs are involved.

Don’t wait to see a doctor if you feel sick or if you don’t get better with over-the-counter medicine. Both types of infections can spread, which is why getting care early is important. If you’re not sure why you feel sick, that’s what your care team is there for.

1. MedlinePlus. Bacterial infections. September 27, 2024.
2. MedlinePlus. Viral infections. November 8, 2023.
3. Centers for Disease Control and Prevention. Preliminary estimated flu disease burden 2023–2024 flu season. November 20, 2024.
4. American Lung Association. Antivirals vs antibiotics: What you need to know. December 7, 2024.
5. Centers for Disease Control and Prevention. About antimicrobial resistance. January 31, 2025.
6. American College of Emergency Physicians. Fever. Accessed October 6, 2025.