Pus and infection are generally present at the same time because pus is created as the body tries to fight the bacteria causing the infection. When the body's systems recognize an infection, large numbers of white blood cells are produced to fight it. One type of white blood cell called neutrophils leaves the blood and attacks bacteria in the wound itself. As both neutrophils and bacteria die, they combine with dead skin cells to form the pus that is usually found in an infected wound.
When pus is visible inside a wound or draining from it, that is a sign that infection is present. The body's immune system is doing its job, sending neutrophils to absorb and ingest the dangerous bacteria. This kills the neutrophils, which die and make up most of the cells found in pus. Neutrophils are also one of the main reasons for swelling after an injury — they are pumped into the area to start fighting bacteria right away.
The causes of pus and infection in something like a pimple are basically the same things that cause them to appear in large wounds. In the case of small acne pimples, the body is fighting the bacteria that clogged the pores. This creates the pus that is usually visible under the skin as a whitehead. The infection may be minor, but the same neutrophils causing the whitehead would also fight bacteria in a large wound, such as a cut or burn.
A risk of infection is present anytime the skin is broken. Generally, larger and more severe wounds have a higher risk of pus and infection because they take longer to heal than smaller ones. Even a tiny break in the skin can leave a person open to infection if the right bacteria get in. Human skin is covered in millions of bacteria, but most of them present no danger. Some, however, such as different types of Staphylococcus and Streptococcus, are prone to causing bacterial infections.
Typically, pus is thick and may have an unpleasant odor. The color of pus is usually white to yellowish-white, though in some cases, it can appear slightly green or dark. Pus cells trapped under the skin are generally referred to as a pimple or a boil. When the skin is opened, the pus can be drained out to help promote healing. Some wounds will drain on their own, while some need to be opened to be cleaned and treated.
People who have pus and infection present should take care to keep the area clean to avoid spreading the bacteria. Minor infections such as pimples or tiny cuts can generally be treated at home. Washing with soap and water, and using such treatments as alcohol or antibiotic ointment can help kill bacteria and speed healing. In the case that pus and infection does not heal quickly or that a wound is large and creates a lot of pus, a doctor's treatment is warranted. A doctor can help keep the bacteria from spreading into the bloodstream where it can become a much more serious condition.