Kaposi sarcoma (KS) has traditionally appeared when the immune system is severely weakened, most often at CD4 counts below 200 cells per cubic millimeter. That threshold still holds for the majority of cases, but the pattern has shifted over time. In the most recent data available, more than a third of KS cases diagnosed between 2002 and 2008 occurred at CD4 counts of 350 or higher, meaning this cancer can show up well before someone reaches the advanced stages of HIV.
The CD4 Count Connection
CD4 cells are the immune cells HIV destroys over time. The lower your count drops, the more vulnerable you become to infections and cancers your immune system would normally suppress. KS risk climbs steeply once CD4 counts fall below 350, with the highest rates occurring under 200. People with counts below 350 have roughly eight times the risk of developing KS compared to those with higher counts.
That said, KS is not exclusively a late-stage disease. In the early years of the epidemic, about 18% of cases appeared at CD4 counts above 350. That proportion dropped to 7% in the mid-1990s when antiretroviral therapy (ART) became available, then rose again to 35% in recent years. The likely explanation: as more people start treatment earlier, a higher share of KS cases now occur in people whose immune systems haven’t yet deteriorated dramatically. This means you can’t assume a relatively healthy CD4 count makes KS impossible.
What Triggers Lesion Formation
KS is caused by a second virus called human herpesvirus 8 (HHV-8), which many people carry without symptoms. When HIV weakens the immune system, HHV-8 can reactivate and spread through the bloodstream into tissues. The process starts with immune dysfunction that activates inflammatory signals. Those signals cause blood vessel cells to change shape, grow abnormally, and form new blood vessels, which is why KS lesions look so deeply colored.
In its early stages, KS behaves more like a reactive, inflammatory process than a traditional cancer. The lesions are polyclonal, meaning they arise from multiple cells rather than a single mutated one. They can even regress if immune function recovers. Over time, however, especially if immunosuppression continues, KS can progress into a true malignancy.
Both a high HIV viral load and a low CD4 count independently raise KS risk. The ratio of CD4 to CD8 cells also plays a role. This means uncontrolled HIV replication is a driver of KS even before CD4 counts hit critically low levels.
What the First Lesions Look Like
The earliest sign is usually a skin lesion that appears purple, red, or brown. These spots can be flat or slightly raised and range from a few millimeters to several centimeters. They most commonly show up on the feet, legs, and face, though they can appear anywhere. Some people develop a single spot; others have lesions in multiple areas at once.
KS lesions progress through three stages. They start as flat patches (the patch stage), thicken into raised plaques (the plaque stage), and can eventually grow into firm nodules (the nodular stage). The pace of this progression varies widely. In someone with very low immune function, it can happen over weeks. In others, early patches may remain stable for months.
Lesions Inside the Mouth
The mouth is one of the most common non-skin locations for KS and sometimes the very first place it appears. About half of oral KS lesions develop on the hard palate (the roof of the mouth), followed by the gums at roughly 28%. They can look like flat, dark purple or violet spots, or grow into larger, bumpy nodules.
Oral KS is sometimes the clue that leads to an HIV diagnosis. A person may visit a doctor or emergency room for other symptoms and have violaceous (deep purple) spots noticed during a routine oral exam. These lesions are painless in early stages, which means they can go unnoticed for some time.
When KS Spreads to Internal Organs
KS does not always stay on the skin. It is the most common gastrointestinal cancer in people with AIDS, found in roughly 40% of patients. Among those who already have skin lesions, more than half also have KS in the digestive tract. Internal KS is often completely silent, producing no symptoms at all. When it does cause problems, the symptoms include abdominal pain, weight loss, nausea, diarrhea, or bleeding from the mouth, esophagus, stomach, or colon. In rare cases, it can lead to bowel obstruction or perforation.
A Paradox After Starting Treatment
One counterintuitive pattern catches many people off guard: KS can appear or worsen shortly after starting ART. This happens because recovering immune cells mount a sudden inflammatory response against HHV-8 already present in the body. This reaction, called immune reconstitution inflammatory syndrome (IRIS), typically occurs within the first two months of treatment, though it can happen anywhere from a few days to six months after starting ART.
There are two forms. “Unmasking” IRIS reveals KS that was already developing but hadn’t become visible yet, with a median onset around 21 days after starting treatment. “Paradoxical” IRIS causes existing KS lesions to suddenly flare, with a median onset around 11 days. Both can occur even in people whose immune suppression was relatively mild. This flare is temporary and does not mean treatment is failing. It reflects the immune system waking up and reacting to what it finds.
How KS Is Confirmed
A tissue biopsy is the gold standard for diagnosing KS. Under the microscope, the hallmark is spindle-shaped cells, which are present at every stage. The definitive test looks for a protein called LANA, produced by HHV-8 inside infected cells. A positive LANA stain in the right clinical context confirms the diagnosis. Early-stage biopsies show abnormal small blood vessels growing through tissue, while later-stage biopsies reveal dense clusters of spindle cells and even more blood vessel growth.
The Bigger Picture Today
Globally, about 70% of all Kaposi sarcoma cases are attributable to HIV. In 2022, an estimated 24,500 cases of HIV-related KS were diagnosed worldwide, making it one of the three cancers most strongly linked to the virus. The burden falls hardest on sub-Saharan Africa, where rates of HIV-attributable cancer reach 27.6 per 100,000 people in southern Africa compared to just 0.2 per 100,000 in Asia.
Early and consistent ART remains the most effective way to prevent KS. Keeping HIV viral load undetectable and CD4 counts high suppresses HHV-8 reactivation and dramatically lowers risk. But because KS can appear at higher CD4 counts than it used to, any new purple, red, or brown skin lesion or unexplained oral discoloration in someone living with HIV warrants prompt evaluation, regardless of how well treatment seems to be working.