Is Pus a Sign of Infection? What Causes It?

Medically Reviewed on 4/18/2022

What is pus?

Pus is a thick fluid that builds up when an injury becomes infected. But pus isn't the only sign of an infection.
Pus is a thick fluid that builds up when an injury becomes infected. But pus isn't the only sign of an infection.

The word “pus” conjures all kinds of unpleasant images. It’s linked with injuries and infection and no one likes to think about that.

Pus is a thick fluid that builds up when an injury becomes infected. It can be white, yellow, green, or brown. It may have a foul odor but that is not always the case. It may have no scent at all.

But pus is a natural part of the healing process for wounds. Pus is a sign that a wound is infected but it is also a sign that your body is trying to fight the infection and heal the injury.

Once an infection has started, your immune system begins trying to fight it off. It sends white blood cells to the area to destroy the bacteria. The white blood cells wear out and die after a while, as does some of the injured tissue. The residue collects in the injured area and becomes what we call pus.

Learn more about pus, why it appears, and how to treat an infected wound.

Signs of pus and wound infection

Pus isn’t the only symptom of an infection. The area will usually look red and swollen. It may be warm to the touch and the infected spot could be painful. If the situation gets worse, you might notice symptoms like fever, aches, or chills as your body fights the infection.

Causes of pus and wound infection

Pus often shows up in an abscess, a space that develops when there is a breakdown in your body’s tissue. When the outer layers of skin break for any reason, bacteria can enter the wounds and cause an infection. Abscesses can occur on the body, including the skin, mucus membranes, or your internal organ.

Skin abscesses are very common. Anything from a small cut to an ingrown hair can be a point of entry for bacteria. Sometimes these small wounds get infected and come to a head, similar to a pimple. There will be a pocket of pus visible on top of a red, painful bump.

Eventually, the abscess will rupture and drain, which helps the healing process.

Infections can also happen below the skin. When that happens, you may notice a swollen, red area that is painful to the touch. You can’t see it, but pus is collecting in the abscess. If you don’t get treatment, the infection can worsen or spread.

Surgical wounds are also at risk for infections. You may notice new or worsening pain around the incision if it gets infected. Swelling and redness are signs of infection, along with any pus that soaks through bandages or comes out in a drainage tube.


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When to see the doctor for pus and wound infection

If the infection is small — less than half an inch across — you can probably treat it at home with warm compresses.

You should see a doctor if you have a sore that is larger than half an inch. Also, call the doctor if a small wound gets bigger or more painful or if you have a fever. You should also see a doctor if the area develops red streaks, which might mean the infection is spreading.

Any infection in the groin area or near your anus needs medical attention as well.

For surgical site infections, you should call the doctor who did your operation. Call a dentist if you have an abscess under a tooth.

Diagnosis of pus and wound infection

When you see your doctor, they will want to know how the injury happened and how long you have had the infection. They will examine the wound and the surrounding area to get an idea of how large the infection might be.

Your doctor might also collect some of the pus from the infection and test it for bacteria or fungus. The test results will help them prescribe medication to clear up the infection.

Treatments for pus and wound infection

Pus-filled abscesses often need to be drained in order to heal completely. Your doctor will decide how to do that based on where the abscess is on your body. It could be something the doctor can do in the office or you might need a more extensive procedure.

Your doctor will use a local anesthetic to numb the area, then drain the pus. They might use a needle to draw it out. They might also make an incision and drain it through that opening.

Dental abscesses can be treated in the office. Your dentist will numb the area and clean out the infection. You will probably need a root canal to solve the problem.

After the pus is cleaned out of the wound, your doctor may give you antibiotics to prevent it from coming back. You should take any medication as directed. You should also follow all the instructions your doctor gives you about caring for the wound when you get home.

Bacterial infection examples

Bacteria are microbes that are invisible to the naked eye because of their size. You can observe them under a microscope.

There are two types of bacteria:

  • beneficial (or good) bacteria and
  • harmful (or bad) bacteria.

The beneficial ones stay in the body and help in digestion, make some vitamins, and fights off infection. The harmful bacteria are found in the environment and can enter your body when you come in contact with them, causing diseases when your immunity is weak. These cause infections and hence are also referred to as infectious bacteria.

How severe is a bacterial infection depends on what types of bacteria are involved. Bacteria most commonly infect the gut, skin, and respiratory system including the lungs, urinary tract, and vagina. There are more than a hundred bacterial infections, but the most common examples are as follows:

  • Bacterial infections of the digestive tract: Food poisoning, also known as Salmonella poisoning, is most often caused by the non-typhoidal Salmonella bacteria found in the intestinal tracts of humans and other animals. It mostly occurs after eating undercooked poultry. The infection causes bloatingdiarrhea, and vomiting. Gastrointestinal (GI) infections can also be caused by Escherichia coli (E. coli). The beneficial strain lives in the gut, but the harmful strain causes infection in the stomach and intestines. The infection usually goes away on its own, but sometimes, it can be life-threatening. The common symptoms are abdominal crampsvomiting, and diarrheaE. coli bacteria commonly spread through contaminated food and sometimes hand-to-hand contact. Infection of the stomach and intestine with Helicobacter pylori (H. pylori) can lead to chronic gastritis and gastric ulcers. These disorders are mostly chronic ones and can cause severe abdominal pain and weight loss. Certain foods are known to trigger the symptoms of chronic gastritis and ulcers.
  • Bacterial infections of the lung: Tuberculosis is a highly communicable or infectious disease caused by the Mycobacterium tuberculosis (M. tuberculosis) bacteria. it may cause chronic cough for months and fever during evenings; sometimes, cough blood, and many times, there is a drastic weight loss, giving an emaciated look. Bacterial pneumonia is a lung infection that can be caused by any of the bacteria: Streptococcus pneumoniae, Klebsiella pneumoniaePseudomonas aeruginosa, and others. The bacteria get transmitted from the infected person to a healthy person through coughing or sneezing. The condition causes high-grade fever, severe coughing, and shortness of breath.
  • Bacterial infections of the vagina and urinary tract: Bacterial vaginosis is an infection of the vagina that can lead to an itchy vagina, a greyish sticky vaginal discharge, and painful urination. The beneficial and harmful bacteria make up the normal vaginal flora. If the number of harmful bacteria increases in the vagina, it leads to bacterial vaginosisUrinary tract infections are most commonly caused by bacteria. They are more common in women than in men. These include cystitis (infection of the urinary bladder) and urethritis (infection of the urethra). The common symptoms include burning or painful urination, frequent trips to the bathroom, and sometimes abdominal painGonorrhea is the second most commonly reported bacterial sexually transmitted infection in the United States. It is caused by the bacteria Neisseria gonorrhoeae and infects both men and women. It most often affects the urethra, rectum, or throat and can also affect the cervix in women.
  • Bacterial infections of the skin: Common bacterial skin infections include:
    • Cellulitis (swollen and red skin that is typically painful and warm to the touch)
    • Erysipelas (similar to cellulitis but that affects the upper or superficial layer of skin)
    • Impetigo (red sores on the face that burst and crust over, especially around a child's nose and mouth and on the hands and feet)
    • Folliculitis (small red or white bumps around hair follicles)
    • Furuncle (boil)
    • Carbuncles (cluster of boils)

Vibriosis (non-cholera) is a bacterial infection that infects wounds. It can be caused by any of the Vibrio species that live in warm seawater: Vibrio vulnificus, Vibrio parahaemolyticus, Vibrio vulnificus, and Vibrio alginolyticus. People catch the infection by consuming raw or undercooked shellfish.

Medically Reviewed on 4/18/2022
Merriam Webster: "Pus."

National Health Service UK: "Abscess."

National Health Service UK: "Dental Abscess."

ScienceDirect: "Pus."

The Ohio State University Wexner Medical Center: "Six signs your wound isn't healing right."

U.S. National Library of Medicine: "Surgical wound infection - treatment."

Medscape: "Infectious Diseases."

CDC: "Diseases and Organisms in Healthcare Settings."