Those most at-risk of contracting measles are under- or unimmunised people, especially children aged under five. Areas of conflict or high-density settings – such as refugee camps – with low immunisation rates and weak local health systems often see epidemics. The Democratic Republic of Congo, Central African Republic and other countries are prone to outbreaks because local health authorities often lack the resources to carry out routine vaccination. In addition, the remote, difficult terrain in some parts of these countries makes the logistics of transporting materials needed for vaccination campaigns extremely challenging.

Measles is caused by a virus so contagious that 90 per cent of non-immune people living with an infected person will catch it; one sick person can infect up to 18 others. It is transmitted from the nose, mouth or throat of infected people through coughing, sneezing and breathing. Vaccination is the best protection against measles.

Measles symptoms appear between 10 and 14 days after exposure to the virus and include a runny nose, cough, eye infection, rash and high fever. In up to 20 per cent of cases, measles causes serious and potentially fatal complications, including severe diarrhoea, respiratory tract infections like pneumonia, blindness and encephalitis (inflammation of the brain).

There is no specific antiviral treatment for measles and people receive treatment for the symptoms, such as fever. Care involves isolating patients and treating them for complications. Most people recover within two to three weeks, but in low-resource settings, up to 15 per cent of people infected with measles die from one or more of these complications. Children, especially those under five and those who may have other immune-suppressing illnesses, such as severe malnutrition, make up most of those who die from the disease.

To declare an outbreak, laboratory confirmation of diagnosis is required. In overcrowded or closed settings, such as camps for refugees and displaced people, one confirmed case of measles is considered an outbreak because the disease spreads so quickly and easily. MSF responds during outbreaks by providing treatment to people with measles and by implementing large-scale vaccination campaigns.