You don’t “catch” psoriasis. It’s a chronic immune system condition that develops when your genetics, immune system, and environmental triggers intersect in a specific way. Around 43 million people worldwide live with psoriasis, and roughly 5.1 million new cases were diagnosed in 2021 alone. Understanding what actually causes it, and what sets it off, helps explain why some people develop it and others don’t.
What Happens Inside Your Skin
Psoriasis starts with your immune system, not your skin. In healthy skin, your body constantly sheds old skin cells and replaces them over the course of about a month. In psoriasis, immune cells called T-cells become overactive and attack healthy skin as though it were an infection. This floods the area with inflammatory signals that force skin cells to reproduce far faster than normal, piling up into the thick, red, scaly patches that define the condition.
The cycle is self-reinforcing. Specialized immune cells in the skin release a signaling molecule that pushes T-cells to produce inflammatory proteins. Those proteins, in turn, recruit more immune cells and stimulate skin cells to multiply even faster. This creates a feedback loop: inflammation drives skin buildup, and the rapid skin turnover keeps fueling inflammation. That’s why psoriasis is chronic. Your immune system essentially gets stuck in “on” mode in the affected areas.
Genetics Set the Stage
The strongest predictor of developing psoriasis is having a close family member who has it. If one parent has psoriasis, your risk increases significantly. If both parents have it, the risk climbs further. Multiple genes are involved, most of them related to immune function. But genetics alone aren’t enough. Many people carry psoriasis-related genes and never develop the disease, which is why triggers matter so much.
Psoriasis onset follows a bimodal pattern, meaning it tends to appear at two distinct life stages. The first and more common peak hits in the late teens to early twenties (around age 17 to 21, depending on sex). The second peak occurs later, around ages 57 to 62. Early-onset psoriasis tends to be more strongly linked to family history and is often more persistent.
Infections Can Trigger the First Flare
For many people, the very first outbreak of psoriasis follows an infection. Strep throat is the most well-established trigger, particularly for a form called guttate psoriasis, which appears as small, drop-shaped spots across the torso and limbs. Research from Karolinska Institutet found that a strep infection alters the behavior of neutrophils, the most abundant type of immune cell. During the infection, a subset of these neutrophils develops the unusual ability to present fragments of the strep bacteria to T-cells. These modified neutrophils then accumulate in the skin, where they activate T-cells and spark inflammation. In someone genetically predisposed, this is enough to launch the first psoriasis flare.
Other infections, including upper respiratory viruses and HIV, have also been linked to new or worsening psoriasis, though the strep connection is the best studied.
Skin Injuries and the Koebner Response
If you already have psoriasis, or carry a strong genetic predisposition, physical damage to the skin can trigger new plaques at the injury site. This is called the Koebner phenomenon. Any wound that penetrates through the outer skin layer into the deeper tissue can do it: cuts, scrapes, surgical incisions, tattoos, piercings, insect bites, sunburns, and even prolonged scratching. The new lesions look identical to existing psoriasis plaques and typically appear days to weeks after the injury.
This is one reason dermatologists advise people with psoriasis to protect their skin from unnecessary trauma, including being cautious about tattoos and managing bug bites without excessive scratching.
Medications That Can Set It Off
Several common drug classes are known to trigger psoriasis in people who’ve never had it, or to make existing psoriasis significantly worse. The ones with the strongest evidence include:
- Beta-blockers (prescribed for high blood pressure, anxiety, and certain heart conditions) can trigger new plaques or convert mild psoriasis into more severe forms.
- Lithium (used for bipolar disorder) causes psoriasis-like skin reactions in an estimated 3 to 45 percent of patients taking it.
- Antimalarial drugs like chloroquine and hydroxychloroquine are more likely to worsen existing psoriasis than to cause it from scratch.
- NSAIDs (common pain relievers) have been implicated, with naproxen the most frequent culprit. In one study, topical indomethacin worsened psoriasis in 14 out of 20 patients with existing disease.
- Tetracycline antibiotics triggered flares in about 4 percent of psoriasis patients in one study.
If you develop new skin symptoms after starting a medication, that’s worth bringing up with your prescriber. In many cases, switching to a different drug in the same class resolves the skin problem.
Obesity, Smoking, and Alcohol
Lifestyle factors don’t cause psoriasis on their own, but they meaningfully raise the odds of developing it. Obesity (a BMI of 30 or above) is associated with roughly double the risk of psoriasis onset compared to a normal weight. Even modest weight gain matters: for every single-point increase in BMI, risk rises by about 9 percent. Excess fat tissue is metabolically active and produces inflammatory signals that overlap with the same pathways driving psoriasis.
Smoking increases the risk of developing psoriasis by about 70 percent. The link is dose-dependent, meaning heavier smokers face greater risk. Alcohol consumption is also associated with more frequent and more severe flares, likely through its effects on both liver function and systemic inflammation.
Stress as a Trigger
Psychological stress is one of the most commonly reported triggers for both initial onset and recurring flares. Stress hormones directly influence immune cell behavior, and chronic stress shifts the immune system toward the kind of inflammatory response that worsens psoriasis. Many people with psoriasis describe a clear pattern: a stressful life event followed, weeks later, by a flare. The relationship also works in reverse. Living with visible psoriasis creates psychological distress, which can perpetuate the cycle.
The Gut Connection
People with psoriasis consistently show different gut bacteria profiles compared to healthy individuals. Beneficial bacteria that produce anti-inflammatory compounds are reduced, while certain pro-inflammatory species are overrepresented. This imbalance can weaken the intestinal lining, creating what’s sometimes called “leaky gut.” When the gut barrier is compromised, bacterial fragments and their byproducts enter the bloodstream, where they activate immune cells and trigger inflammatory cascades that reach the skin.
The relationship is bidirectional. Gut imbalances promote skin inflammation, and the systemic inflammation from psoriasis may further disrupt gut health. Researchers have found that specific beneficial bacteria normally produce short-chain fatty acids, particularly butyrate, which help keep immune responses in check by supporting the development of regulatory T-cells. When those bacteria are depleted, the immune system loses one of its natural brakes. This is a rapidly evolving area of science, and gut-targeted therapies for psoriasis are being actively explored.
Putting It Together
Psoriasis develops through a combination of genetic susceptibility and one or more triggers that push the immune system past a tipping point. For one person, that trigger might be a strep infection at age 19. For another, it might be weight gain and a stressful job at age 55. The underlying genetic wiring determines vulnerability, but the environmental factors determine timing and severity. This is why psoriasis can seem to appear “out of nowhere” after years of normal skin, and why flares come and go in response to life circumstances, illness, and overall health.