A newborn's mysterious rash stumped ER doctors for days. It turned out to be monkeypox.

• A baby contracted monkeypox in August, likely from contact with an infected caregiver.
• The two had "daily close contact" for several weeks before the child showed any signs of infection.

In August, a two-month-old baby showed up at a Florida emergency room with a raised, red rash present on its arms, legs, and trunk.

Doctors weren't sure what was going on with the child, and their routine tests for bacterial causes of the rash, as well as viruses including chickenpox, HIV, and herpes all came back negative.

When the baby returned to the ER two days later, that rash — now a week old — had developed into pox-like lesions all over the infant's body, and many of the bumps had a distinctive dot in the center of them.

This must be molluscum contagiosum, the providers thought. It's a relatively common childhood infection transmitted via skin to skin contact, which doesn't typically require much treatment.

But over the next few days, the baby's bumps began to fill with pus. The pox spread over the child's back, the soles of their feet, face, and eyelids. The providers decided to perform a different test, which confirmed, 10 days after the rash initially surfaced: this is actually monkeypox.

The case study, described in a new report from the Centers for Disease Control and Prevention (CDC) on Monday, provides vivid new details about how monkeypox can develop and present on a newborn baby's body. Analyzing the case, CDC officials explain how the newborn contracted the virus, and emphasize that this is a rare event in the current outbreak.

The child was treated with antivirals and antibodies

The infant, whose name and gender were not published to protect the child's identity, received both the antiviral drug tecovirimat (TPOXX) and an IV drip of disease-fighting antibodies (called immune globulin, or IVIG).

The child did not develop a fever, "tolerated the treatments well, and fully recovered," CDC disease investigators said Monday in their new report.

Pediatric monkeypox infections like this one have been exceedingly rare in the current outbreak. Only 27 monkeypox infections have been diagnosed nationwide in children under the age of 16 years old since the US monkeypox outbreak began in the spring — that's a tiny fraction of the nearly 23,500 monkeypox cases that have been officially documented so far.

What 'close contact' really means: daily care, holding, and sleeping side by side

In this case, the baby likely got monkeypox from very close, sustained, daily contact with one of its four main caregivers.

The man, identified only as "Caregiver B" in the new CDC report, had developed a fever, blood in his urine, as well as a rash visible on his body during the three week period before the baby developed any symptoms. During that time, the baby shared bed linens with Caregiver B, slept alongside him, and had sustained, skin-to-skin contact from being held and cared for every day.

Caregiver B tested positive for an orthopox virus (that's likely also monkeypox) two days after the baby tested positive.

As Dr. Daniel Pastula, a professor of neurology, infectious disease and epidemiology at The University of Colorado and Colorado School of Public Health, previously told Insider, monkeypox can spread between people who are in very close physical contact with one another, whether the contact is sexual in nature or not.

None of the baby's three other caregivers got sick with monkeypox. They all received the Jynneos vaccine, and as of 22 days later, none of them have contracted the virus.