A baby who was dying of rabies passed on the disease to his father and brother by biting them.
The child in the Democratic Republic of the Congo infected his seven-year-old brother and 27-year-old father during an outbreak which affected 154 people.
Both the father and brother recovered after being treated by medical charity Médecins Sans Frontières (MSF) which says at least 10 people have died during the outbreak.
It is very rare for rabies to be transmitted between people - most people who become infected do so by being bitten by an infected dog.
Dr Jantina Mandelkow, who leads MSF's team in Lemera, which is in the DRC's South Kivu region, said: "I held my breath when I heard that a child dying of rabies had bitten and infected his father and brother - it was horrific.
"It's an impossible situation. Family members would obviously want to be with those who are ill, but when a person has rabies they can be a danger to people around them.
"I've only seen one other case of rabies in my career as a doctor and it was one of the worst things I have ever seen.
"Rabies leads to death without treatment, and people in Lemera were incredibly scared - many walked for days to get to the hospital for treatment.
"With vaccinations unavailable elsewhere, they had nowhere else to turn."
Rabies is a very serious viral infection that attacks the brain and nervous system - you can catch rabies if you are bitten by an infected animal. It is almost always fatal unless treated very early.
Symptoms include tingling and itchiness at the site of the infection, fever, an irrational fear of water and aggressive behaviour.
Treatment usually involves cleaning the site of contamination and administering a course of the rabies vaccine in an attempt to prevent the infection spreading to the brain and nervous system.