A new report in the New England Journal of Medicine offers a glimpse of the depth of mystery shrouding MERS. A team of Saudi Ministry of Health researchers describe a November outbreak in a Riyadh urban household of 28 extended family members, four of whom — all men — contracted MERS.

The cluster of cases in this family presents a list of mysteries: Why were all the sick and dead men? With 28 people in this three-building urban household, why were these four infected, and the other 24 spared? The family lived in a big city, had no animals, ate supermarket food and had jobs that offered no contact with the virus. How did they catch MERS?

Until researchers can determine what animal is the natural host of the virus, and how MERS spreads from the host to humans, each new outbreak is dangerous and mysterious. The science is still unfolding.