US Centres for Disease Control scientists study infectiousness of mild Mers cases

Scientists leading the fight against Middle East respiratory syndrome (Mers) say the next critical front will be understanding how the virus behaves in people with milder infections, who may be spreading the illness without being aware they have it.

Authorities in Saudi Arabia reported five new deaths from the Mers respiratory virus, bringing the death toll in the world’s worst-hit country to 168.

It is becoming increasingly clear that people can be infected with Mers without developing severe respiratory disease, said Dr David Swerdlow, who heads the Mers response team at the US Centres for Disease Control and Prevention (CDC).

"You don't have to be in the intensive care unit with pneumonia to have a case of Mers," Swerdlow said. "We assume they are less infectious [to others], but we don't know."

The CDC has a team in Saudi Arabia studying whether such mild cases are still capable of spreading the virus.

Cases of the disease, which causes coughing, fever and sometimes fatal pneumonia, have nearly tripled in the past month and a half, and the virus is moving out of the Arabian peninsula as infected individuals travel from the region.