A study presented at the 15th British HIV Association (BHIVA) conference showed that superinfection—infection with a second stain of HIV in a person who is already positive—could cause serious medical problems, aidsmap reports.

According to the article, the study enlisted eight HIV-positive men who have sex with men (MSM) who were not on antiretroviral treatment and had reported increases in their viral load. Two of the men’s viral loads had risen because they were infected with a second strain of HIV.

The first patient contracted a second, drug-resistant strain of HIV five months after his initial infection. The patient also experienced acute HIV symptoms during the second infection and was then hospitalized for meningitis and a sizeable, but temporary decrease in his CD4 count. The second patient was infected with a second strain three years after his initial diagnosis. His viral load shot up from 3,000 to half a million, but he maintained a CD4 count above 1,000. A year later, his viral load returned to 3,000 again.

Researchers conclude that HIV-positive people who engage in high-risk sexual behavior are susceptible to superinfection “both in the early and established phases of the disease.” This casts doubt on earlier studies indicating that superinfection occurs only in those newly infected with an initial strain of the virus.

They recommend that positive people who are not on treatment who experience sudden increases in viral loads be screened for superinfections. However, the type of screening used in the study is solely a research tool—and it costs nearly $15,000 (£10,000) per patient.