SAUDI ARABIA crossed a grim threshold Wednesday, announcing that the number of cases of Middle East respiratory syndrome, caused by a novel coronavirus, has exceeded 500 in the kingdom, with 157 deaths among them. The global number of MERS cases is now more than 570, with two of them recently reported in the United States.

At the same time, the World Health Organization declared that the disease has not yet reached the point of a “public health emergency of international concern.” The reason is that experts have not yet detected “evidence of sustained human-to-human transmission” of the kind that could lead to a pandemic. Clearly, there has been some transmission involving people in close contact with victims, such as relatives and health-care workers, but not rapidly and broadly.

The WHO decision should not lead to a sense of complacency; the announcement was studded with warnings.

Far too much is still unknown about this virus and how it works. For example, it appears that one reason for the escalating number of cases in Saudi Arabia in recent weeks was a breakdown in infection control. A visiting WHO team found lapses in such basics as hand washing, gloves and masks. Hospital conditions were “suboptimal.” How did the virus spread? Does it survive in the environment? That’s an important factor in infection control for any hospital — whether in Jiddah or Orlando — that hasn’t been answered.

Yet another big information gap has been the lack of a case-control study, which would compare those infected with a similar group of people that is not. Such a study could go a long way toward helping explain how the virus behaves and the risks of transmission and infection.

Comparing older genetic blueprints with recent ones has led experts to conclude there have not been modifications in the genome of the virus that would allow it to be transmitted from human to human more efficiently than before. That’s reassuring, but the key will be a steady stream of genetic sequencing information to keep watch on the unpredictable evolutionary process.

The WHO emphasized that its concern has significantly increased in part because of “critical gaps in information.” That is putting it politely. A large share of the responsibility for this black hole must fall on Saudi Arabia, a closed society where the disease was not met with sufficient urgency for more than a year. While there are now signs of more openness, the world’s toolkit for fighting MERS remains rather bare. There is no vaccine or effective antiviral. The WHO called for speeding up critical investigations, including case-control, serological, environmental and animal studies. The kingdom must do everything it can to expedite this vital research.

Right now, MERS seems like a nightmare far away from the United States. But it is not. Both infections arrived here being carried by air passengers who passed by hundreds of other people on their journey. The best defense today is a deeper understanding of the virus and its behavior — and the sooner, the better.