A new study suggests a large number of MERS infections are going undetected.
The researchers estimate that for each case of the disease that has been found, five to 10 may have been missed.
They say it's not a case of countries trying to hide cases of the disease, which is formally known as Middle East respiratory syndrome.
They argue that the problem relates to who affected countries are testing.
Experts have been worried that testing that focuses on people who show up sick at hospitals will miss patients whose symptoms don't require that level of care.
In Saudi Arabia, where the most cases have occurred, surveillance is focused on patients who require intensive care.
That would filter out even more potential cases.
The study was done by scientists from Imperial College London, the University of Edinburgh and the Institute Pasteur in Paris.
It is being published in the journal Lancet Infectious diseases.
To date there have been about 155 confirmed MERS cases and at least 65 of those infections have been fatal.
