Bacterial and Viral Infections

Medically Reviewed by Zilpah Sheikh, MD on June 02, 2024
7 min read

Bacterial and viral infections have many things in common. Both types of infections are caused by microbes -- bacteria and viruses, respectively -- and spread by things such as:

  • Coughing and sneezing
  • Contact with infected people, especially through kissing and sex
  • Contact with contaminated surfaces, food, and water
  • Contact with infected creatures, including pets, livestock, and insects such as fleas and ticks

Microbes can also cause:

  • Acute infections, which are short-lived
  • Chronic infections, which can last weeks, months, or a lifetime
  • Latent infections, which may not cause symptoms at first but can reactivate over a period of months or years

Most importantly, bacterial and viral infections can cause mild, moderate, and severe diseases.

Bacterial and viral infections in the past

Throughout history, millions of people have died of diseases such as bubonic plague or the Black Death (caused by Yersinia pestis bacteria) and smallpox (caused by the variola virus).

In recent times, viral infections have been responsible for three major pandemics:

  • The 1918-1919 Spanish flu pandemic that killed 20-40 million people
  • The ongoing HIV/AIDS pandemic, which has killed over 40.5 million people (as of 2023)
  • The COVID-19 pandemic, which has killed more than 7 million people worldwide as of May 2024

Bacterial and viral infections can cause similar symptoms, including:

These are ways the immune system tries to get rid of infectious organisms.

Although symptoms are similar, bacterial and viral infections are not alike in many other important respects. That's mostly because of the organisms' structural differences and the way they respond to medications.

Although bacteria and viruses are both too small to be seen without a microscope, they're as different as giraffes and goldfish.

Bacteria are relatively complex, single-celled creatures, often with a rigid wall and a thin, rubbery membrane surrounding the fluid inside the cell. They can reproduce on their own. Fossilized records show that bacteria have existed for about 3.5 billion years and can survive in different environments, including extreme heat and cold, radioactive waste, and the human body.

Most bacteria are harmless, and some actually help by digesting food, destroying disease-causing microbes, fighting cancer cells, and providing essential nutrients. Fewer than 1% of bacteria cause diseases in people.

Viruses are tinier; the largest of them are smaller than the smallest bacteria. All they have is a protein coat and a core of genetic material, either RNA or DNA. Unlike bacteria, viruses can't survive without a host. They can only reproduce by attaching themselves to cells. In most cases, they reprogram the cells to make new viruses until the cells burst and die. In other cases, they turn normal cells into malignant or cancerous cells.

Also unlike bacteria, most viruses do cause disease, and they're quite specific about the cells they attack. For example, certain viruses attack cells in the liver, respiratory system, or blood. In some cases, viruses target bacteria.

Common bacterial infections

You can get bacterial infections in many parts of your body, including your gut, skin, sinuses, and urinary tract. Common bacterial infections include:

  • Strep throat
  • Lyme disease, which is spread by ticks
  • C. diff, which affects your intestines
  • Salmonella and campylobacter, which cause food poisoning
  • Impetigo, boils, and cellulitis (these affect your skin)
  • E. coli, which causes many urinary tract infections (UTIs)
  • Chlamydia and gonorrhea, which are sexually transmitted
  • Pneumococcal disease, which can cause sinus and ear infections, as well as some types of pneumonia
  • Bacterial vaginosis, an excess of bacteria in your vagina

Common viral infections

Viruses most often affect your respiratory and digestive systems, but they can cause problems in other parts of your body.

Viral infections of your respiratory system include:

  • Common cold
  • Flu
  • Human metapneumovirus (hMPV)
  • Respiratory syncytial virus (RSV)
  • Parainfluenza

These viruses can cause bronchitis, ear infections, pneumonia, and other conditions.

Common viral infections of your digestive system include:

  • Rotavirus
  • Norovirus
  • Astrovirus
  • Hepatitis

The first three can cause gastroenteritis, which some people call "stomach flu." Hepatitis affects your liver and can be a chronic condition.

Neurological viruses affect tissue in your brain or spinal cord. They can cause paralysis as well as dangerous swelling of your brain. They include:

  • Polio 
  • Rabies
  • West Nile virus

Congenital viruses pass to the fetus during pregnancy. They can cause vision and hearing loss and developmental delays. They include:

  • Zika
  • Cytomegalovirus (CMV)
  • Rubella

How long do bacterial and viral infections last?

If you have a bacterial infection, it will probably clear up in a week or two with antibiotic treatment. It's important to take all of your medication even if you start to feel better, otherwise, the bacteria might come back.

How long a viral infection lasts depends on what type of virus causes it. Most respiratory viruses clear up in a week or two. Hepatitis can cause chronic health problems that last years. HIV can't be cured and is a lifelong infection, although there are treatments to manage it.

You should consult your doctor if you think you have a bacterial or viral infection. Exceptions include the common cold, which is usually not life-threatening.

In some cases, it's difficult to determine whether an illness is viral or bacterial because many ailments -- including pneumonia, meningitis, and diarrhea -- can be caused by either. But your doctor may be able to determine the cause by listening to your medical history and doing a physical exam.

If necessary, they can also order a blood or urine test to help confirm a diagnosis, or a "culture test" of tissue to identify bacteria or viruses. Occasionally, a biopsy of affected tissue may be required.

There are different treatments for bacterial and viral infections. Antibiotics will not treat a virus.

Treating bacterial infections

The discovery of antibiotics for bacterial infections is considered one of the most important breakthroughs in medical history. Unfortunately, bacteria are very adaptable, and the overuse of antibiotics has made many of them resistant to antibiotics. This has created serious problems, especially in hospital settings.

Treating viral infections

Since the beginning of the 20th century, vaccines have been developed to combat many viruses. Vaccines have drastically reduced the number of new cases of viral diseases such as polio, measles, and chickenpox. In addition, vaccines can prevent infections such as the flu, hepatitis A, hepatitis B, human papillomavirus (HPV), and others.

But the treatment of viral infections has proved more challenging, mainly because viruses are relatively tiny and reproduce inside cells. Antiviral medications have become available for some viral diseases, such as herpes simplex virus infections, HIV/AIDS, and influenza. But their use has been associated with the development of drug-resistant microbes.

Because viruses don't respond to antibiotics, many experts recommend against using antibiotics without clear evidence of a bacterial infection.

Viral and bacterial infections can cause similar symptoms, making it tough to distinguish between the two solely based on symptoms. Your symptoms might affect your respiratory system, digestive system, skin, or other parts of your body. Your doctor may have to do some tests to determine whether your illness is caused by a virus or bacteria. If your infection is bacterial, you'll be treated with antibiotics. But these drugs don't work on viruses. Many viruses can be prevented with vaccines. Viral infections can be treated, in some cases, with antiviral drugs.

How do you know if an infection is viral or bacterial?

It might be hard to figure out what's making you sick. Your doctor may be able to make a diagnosis based on your symptoms. Sometimes, lab results are the only way to know. Your health care provider might take a sample from you and send it to the lab. For bacterial infections, these can include:

  • Urine
  • Poop (stool)
  • Blood
  • Skin
  • Sputum (the stuff you cough up)
  • Fluid that your eyes produce
  • Fluid from around your brain or spinal cord (CNS fluid)

If your internal organs are affected, you might have X-rays, ultrasounds, MRIs, or CT scans.

If your doctor thinks you have a viral infection, you might need an X-ray, ultrasound, MRI, or CT scan to see how your illness is affecting your body. Scans and X-rays can't identify a viral infection, however. To do that, your health care provider may ask a lab to look for the virus itself, antigens, or antibodies in a sample taken from your body. Those samples can include:

  • Blood
  • Spit
  • Sputum
  • Cells from inside your nose
  • Skin
  • Urine
  • Poop (stool)
  • CNS fluid
  • Cells from your cervix taken during a pap smear

Which is more serious, a viral infection or a bacterial one?

Your immune system typically can clear a viral infection easier than a bacterial one. But viruses still can cause life-threatening illnesses, such as COVID.

Is it harder to treat a viral or bacterial infection?

You can treat a bacterial infection with antibiotics. Antiviral drugs are available for some viral illnesses, but they can only speed up your recovery. Viruses have to run their course. Some, such as HIV, never leave your body. The best way is to avoid getting it in the first place. Many viruses can be prevented by vaccines.

Do antibiotics work on viral infections?

No. Antibiotics aren't effective against viruses, and taking an antibiotic when your illness is viral can lead to bacteria becoming drug-resistant. Sometimes, viral illnesses lead to bacterial infections. For instance, a cold can cause fluid to build up in your respiratory system, and fluid in your ear can become infected with bacteria. In that case, an antibiotic will help clear the bacterial infection. But it won't do anything to the virus.