Staph infections are caused by Staphylococcus aureus bacteria, which live harmlessly on the skin or in the nose of roughly 20 to 30 percent of healthy people. An infection develops when these bacteria get past the skin’s protective barrier, typically through a cut, scrape, or other break in the skin, and begin invading deeper tissue.
Understanding what triggers that shift from harmless colonization to active infection helps explain why some people get staph infections repeatedly while others never do.
How Staph Bacteria Live on Your Body
S. aureus colonizes the front of the nasal passages in 20 to 80 percent of the population at any given time, depending on the study and whether someone is a persistent or intermittent carrier. Most carriers never develop an infection. The bacteria sit on the skin surface or inside the nostrils, kept in check by the skin barrier and the immune system. This is called colonization, not infection.
The trouble starts when the bacteria find a way through that barrier. Once inside, S. aureus is remarkably well-equipped to cause damage. Its surface is coated with proteins that bind tightly to human tissue components like collagen and fibrinogen, essentially gluing the bacteria in place. From there, it secretes enzymes that break down surrounding tissue, helping it spread deeper. It can even alter the electrical charge on its outer membrane to resist the natural antimicrobial defenses in your skin and mucous membranes.
The Role of Broken Skin
A break in the skin is the single most common trigger for a staph infection. This can be obvious, like a surgical wound or a deep cut, or subtle, like a small razor nick, a bug bite, or cracked skin from eczema. Staph infections on the skin tend to cluster in areas that are already damaged: patches of chronic skin disease, injection sites for insulin, or places where you’ve been scratching.
Once bacteria enter through the break, they trigger tiny blood clots (microthrombi) at the invasion site. Then they secrete enzymes that help them escape from those clots and push further into the tissue. At the same time, other enzymes disable parts of your immune response, specifically the processes that would normally tag the bacteria for destruction by white blood cells. This combination of clot formation, tissue breakdown, and immune evasion is what allows a minor skin wound to turn into a spreading infection.
How Staph Spreads Between People
Staph bacteria spread through two main routes: direct skin-to-skin contact and contaminated surfaces or objects. You can pick up staph from shaking hands with a carrier, sharing a towel, or using a razor that someone else used. The bacteria can survive on surfaces like towels, razors, gym equipment, and furniture for hours, days, or even weeks.
Common transmission scenarios include:
- Shared personal items: towels, washcloths, razors, and clothing
- Gym and athletic settings: shared equipment, locker room benches, and whirlpool tubs
- High-touch surfaces: desks, chairs, faucets, light switches, and remote controls
- Close physical contact: contact sports, crowded living conditions, and childcare settings
Athletic facilities are particularly high-risk environments. The CDC notes that MRSA spreads fast in gyms, locker rooms, and health clubs because of the combination of shared equipment and regular skin-to-skin contact. Any surface that touches bare skin can serve as a vehicle, especially if someone has an open wound.
Why Staph Is So Good at Causing Damage
S. aureus produces an unusually wide arsenal of toxins and enzymes, which is part of what makes it more dangerous than many other common bacteria. About 95 percent of S. aureus strains carry the gene for a toxin called alpha-hemolysin, which punches holes in human cells by activating a protein on cell surfaces that breaks apart the connections between cells. This compromises the barriers that normally keep bacteria contained, allowing the infection to spread into the bloodstream.
Some strains produce toxins that act as “superantigens,” meaning they hijack the immune system in a dangerous way. Instead of triggering a targeted immune response, these toxins activate massive numbers of immune cells all at once, flooding the body with inflammatory signals. This overreaction can cause high fever, dangerously low blood pressure, and in severe cases, organ failure. This mechanism is what drives toxic shock syndrome.
Other toxins target the gut. Staphylococcal food poisoning happens when bacteria growing in improperly stored food release toxins that disrupt intestinal activity, causing nausea, vomiting, abdominal pain, and diarrhea. These symptoms come from the toxins themselves, not from live bacteria in the gut, which is why food poisoning can hit within hours of eating contaminated food.
Medical Devices and Biofilm
One of the more insidious causes of staph infection involves medical devices: catheters, artificial joints, pacemakers, and other implants. Within minutes of a device being placed in the body, human proteins like fibronectin and collagen coat its surface. S. aureus has specialized surface proteins that recognize and grab onto these coatings, giving the bacteria a foothold on the device.
Once attached, the bacteria build a biofilm, a structured community of bacteria encased in a protective slime layer. Biofilms are extremely difficult for both the immune system and antibiotics to penetrate. On prosthetic joints, these biofilms can grow into large, free-floating clusters in the joint fluid, sometimes visible to the naked eye. Biofilm-related infections are a leading reason why infected implants often need to be surgically removed rather than treated with antibiotics alone.
Who Is Most Vulnerable
Healthy people with intact skin and normal immune function can and do get staph infections, but certain conditions dramatically increase the risk. In community settings, the CDC identifies people with chronic conditions like diabetes, cancer, and vascular disease as being at greater risk, along with people who inject drugs.
In healthcare settings, the highest-risk patients are those in intensive care units, those recovering from surgery, those with implanted medical devices, and those with weakened immune systems. Specific conditions that raise vulnerability include HIV infection, kidney failure requiring dialysis, chemotherapy treatment, and long-term steroid use. Each of these impairs either the skin barrier or the immune cells that would normally contain the bacteria before they can establish an infection.
Neutrophils, the white blood cells that serve as first responders against bacterial invaders, are the most critical line of defense against S. aureus. People with conditions that reduce neutrophil numbers or function, whether from disease or medical treatment, are at significantly higher risk for serious and systemic staph infections.
MRSA: When Antibiotics Stop Working
MRSA (methicillin-resistant Staphylococcus aureus) is caused by the same species of bacteria, but strains that have acquired resistance to an entire class of commonly used antibiotics called beta-lactams. This class includes many of the first-line drugs traditionally used for staph infections.
MRSA was once confined almost entirely to hospitals and nursing homes (healthcare-associated MRSA), but community-associated MRSA now circulates widely among otherwise healthy people. Community strains often carry a toxin called Panton-Valentine leukocidin, which destroys white blood cells and is linked to more aggressive skin infections and, in rare cases, severe bloodstream infections. The causes and transmission routes are the same as for regular staph, but the infections are harder to treat because fewer antibiotics work against them, and the bacteria can persist on surfaces for extended periods.