Pimples form when oil, dead skin cells, and bacteria build up inside your hair follicles and trigger inflammation. It’s an incredibly common process: around 85% of people between ages 12 and 24 experience acne, and it often persists well into adulthood. Understanding what’s actually happening beneath the surface helps explain why breakouts show up when they do and why certain habits make them worse.
Four Things Happen Inside a Pore
Every pimple starts with the same basic chain reaction involving four factors: excess oil, a clogged pore, bacteria, and inflammation. These overlap and feed into each other, which is why acne can be so persistent.
Your skin constantly produces an oily substance called sebum, which normally travels up through hair follicles and spreads across your skin to keep it moisturized. Problems start when your body makes too much of it. Hormones called androgens are the main driver here. When androgen levels rise, your oil glands ramp up production, flooding the follicle with more sebum than it can handle.
At the same time, the cells lining the inside of the follicle can start behaving abnormally. Instead of shedding and clearing out of the pore the way they should, they become sticky and clump together. This creates a tiny plug deep in the follicle, called a microcomedone, that traps all that excess oil underneath. You can’t see a microcomedone yet, but it’s the seed of every pimple, blackhead, and whitehead.
Once the pore is sealed off, a specific type of bacteria that naturally lives on your skin begins to thrive in that oil-rich, oxygen-poor environment. As the bacterial population grows, your immune system responds. White blood cells called macrophages detect the bacteria and release a cascade of inflammatory signals. This immune response is what turns an invisible clogged pore into a red, swollen, painful bump.
Hormones Are the Biggest Trigger
Androgens don’t just control how much oil your skin makes. They also influence how follicle cells develop, which can contribute to that abnormal clogging process. This is why acne peaks during puberty, when androgen levels surge for the first time, and why it flares around menstrual periods, during pregnancy, or with hormonal shifts in your 20s and 30s.
Adult acne is far more common than most people realize, especially in women. Roughly 50% of women in their 20s deal with acne, about 33% in their 30s, and 25% in their 40s. Among men in their 40s, around 12% still report breakouts. These numbers make it clear that acne isn’t just a teenage problem. Any life event or condition that shifts your hormone balance can restart the cycle.
Genetics Play a Major Role
If your parents had acne, your chances of getting it are significantly higher. Twin studies estimate that genetics account for roughly 80 to 85% of the variation in acne severity between people. That means your DNA largely determines how much oil your skin produces, how readily your pores clog, and how aggressively your immune system responds to bacteria. You can influence the other 15 to 20% through behavior and environment, but the baseline is mostly inherited.
What You Eat Can Make It Worse
Diet doesn’t cause acne on its own, but certain foods can amplify the hormonal signals that drive breakouts. Foods with a high glycemic index (white bread, sugary drinks, processed snacks) cause your blood sugar to spike quickly. Your body responds by pumping out insulin, and elevated insulin does two things that matter for your skin: it stimulates androgen production, which increases oil output, and it raises levels of a growth factor called IGF-1 that accelerates skin cell turnover inside follicles. More oil plus faster clogging equals more pimples.
This creates a feedback loop. Androgens boost IGF-1, which in turn stimulates more androgen production, cycling back to even more sebum. Dairy products appear to have a similar effect, likely because milk contains its own hormones and growth factors. One striking piece of evidence: an epidemiologic study from the early 1970s found that Inuit populations in Northern Canada had virtually no reported acne until Western foods like soda, beef, dairy, and processed products were introduced to their diet.
Stress Fuels Breakouts
The connection between stress and acne isn’t just anecdotal. When you’re stressed, your body releases a hormone called corticotropin-releasing hormone (CRH). Beyond its role in the stress response, CRH is active in the skin itself, where it can increase oil production and worsen inflammation. Elevated CRH in the skin can intensify existing inflammatory conditions or even contribute to new ones. This helps explain why breakouts tend to cluster around exams, deadlines, or difficult life events, even if nothing else about your routine has changed.
Why Different Pimples Look Different
Not all breakouts are the same, and the type you get depends on how far the process has progressed and how deep the inflammation goes.
- Blackheads: Open pores where the trapped plug is exposed to air. The dark color comes from oxidation of the material inside, not dirt.
- Whiteheads: Closed, plugged follicles where the contents stay sealed under a thin layer of skin.
- Papules: Red, tender bumps that form when the follicle wall breaks down and the immune system moves in. There’s no visible pus at this stage.
- Pustules: Similar to papules, but the immune response has produced a visible pocket of pus near the surface.
- Nodules: Larger, deeper, more painful lumps that develop when the infection and inflammation push further into the skin.
- Cysts: Deep, fluid-filled lesions beneath the surface. These are the most likely to cause lasting scars.
Blackheads and whiteheads are considered non-inflammatory. They’re clogged pores, but the immune system hasn’t been activated yet. Everything from papules onward involves active inflammation, which is why those types are red, sore, and harder to treat with surface-level products alone.
Why Some People Get Acne and Others Don’t
Given that everyone has oil glands, skin bacteria, and shedding skin cells, it can feel unfair that some people break out constantly while others never do. The difference comes down to the sensitivity of your oil glands to hormones (largely genetic), the rate at which your follicle cells shed or stick together, and how reactive your immune system is to bacteria that are present on everyone’s skin.
Two people can have the same amount of acne-associated bacteria on their face, but only one develops pimples because their immune system mounts a stronger inflammatory response. Similarly, two people with the same androgen levels can have very different oil production because one person’s oil glands have more hormone receptors. These invisible differences, stacked on top of each other, explain why acne severity varies so dramatically from person to person, even within the same family.