People with severely weakened immune systems, such as those infected with HIV, can experience severe symptoms and even die from a monkeypox infection, according to a US study published on Wednesday.

The study analyzed the cases of 57 US patients hospitalized with severe complications of monkeypox.

Almost all (83%) had severely weakened immune systems, in most cases due to human immunodeficiency virus (HIV) infection. Many of these patients were not being treated for the virus that causes AIDS.

More than 90 countries where monkeypox is not endemic have reported outbreaks of this viral disease, which the World Health Organization has declared a global health emergency. Confirmed cases have reached 76,757.

In the United States, just over 28,000 people have been infected with monkeypox since the outbreak began in May. Cases began to peak in mid-August and have dropped sharply since then, helped by the country’s vaccination campaign.

Deaths outside of Africa, where the virus is endemic, are rare, as are those caused by the form of the virus now circulating in the United States, clade IIb.

For the study, health authorities investigated some of the most serious cases of monkeypox, which is spread by close contact with an infected person.

In all, 47 of these people were also found to be infected with HIV, although only four of them were receiving antiretroviral therapy, powerful drugs that keep the virus at bay. The majority (95%) were male and 68% were black.

According to the analysis, 17 patients required care in an intensive care unit, and 12 have died, including five in which monkeypox was a contributing factor or the confirmed cause of death.