The Middle East respiratory syndrome coronavirus (MERS-CoV) is associated with the highest mortality rate among the 6 known human coronaviruses. As of 26 June 2015, 1356 laboratory-confirmed cases have been reported to the World Health Organization, including at least 484 related deaths (ie, average mortality rate, approximately 36%), which is more than 3 times the mortality rate associated with severe acute respiratory syndrome CoV (SARS-CoV)[1]. Why the mortality rate due to MERS-CoV infection is so high is unclear.

In the current issue of The Journal of Infectious Diseases, Hin Chu et al provided a clue to elucidate the high pathogenicity of MERS-CoV. This is a challenging task because the pathogenicity of MERS-CoV is complicated with numerous factors involved. In addition, the lack of human autopsy data further complicates the task. However, there are a number of similarities between the pathologies caused by MERS-CoV and those caused by SARS-CoV, as well as significant differences, as recently reviewed elsewhere [2, 3]. In both cases, the immune system plays a pivotal role, but it is differentially affected by the two viruses.