The deadliest strain of coronavirus, which has infected seven people since November and killed two, comes from a single family, raising the possibility of it infecting relatives or the same person both overseas and in Europe. But while the precise details are not known, there are warning signs and a prediction that the spread could be closely tied to human-to-human transmission.

While it is impossible to predict the exact path that this virus could take, there are several worrisome indicators. Only four people in six months have died of this strain in Europe, so far. This group of cases is “extremely rare,” said Neil Ferguson, an epidemiologist with the World Health Organization. He also said the foreign contacts of the late laboratory workers appear to be limited to people who were either traveling or recently returned from outside the region. There is also no historical evidence of groups of people contracting this strain and spreading it locally.

Dr. Ferguson says other data suggests that since the virus first came to light, it may have been latent in the population in Saudi Arabia, where a portion of the virus arose and has sickened 37 people. “This is a community-level outbreak and every dose so far has been lethal,” he said. That suggests that something else may be at play, some environmental issue such as a chemical pollutant that would lead to virulent disease. The incubation period in this form of the virus, he said, is less than a day.

The same could also be true for those contracting the more common acute coronavirus, which occurs in people who have traveled and returned from Pakistan or Iran. Most of the 24 known cases have been mild and have not spread widely. It does not seem to be related to the outbreak in Saudi Arabia, which is marked by the deadlier form. The rapid spread of this strain through the Middle East was a contributing factor in that outbreak.

“You could argue that these coronaviruses have not crossed a much more important threshold,” Dr. Ferguson said.

It is also very possible that the strain could mutate in a way that would, in theory, make it harder to detect. Dr. Ferguson said it is not unusual for humans to acquire a new virus every year. We do not yet know whether the same strain has been isolated from pigs in the United States, or how it was taken from a hospital in England, where it was picked up from a healthcare worker. “We don’t know enough about this virus to know what it will look like in humans” after developing, he said.

One reason people are worried is that the group that originally developed the infection carried it to countries where it had been previously thought to have not spread: Iran and Pakistan.

Although it is still quite rare, this form of coronavirus has the potential to spread and cause death. If it becomes “wild,” for want of a better word, it will not only affect those traveling to that area of the world, but those who live there.

“Most people know you cannot catch this virus from animals. But it is known you can catch it by coming into contact with blood and organs,” Dr. Ferguson said. “So, you can pick it up in a hospital and do a procedure on a patient and then catch it on your hands while handling the cuts, and then you can touch your mouth, and then you can transmit it in that context.”

The best defense against this disease is going to be stepped-up screening of those who travel. “Monitoring travelers can be the most effective way to detect the disease,” Dr. Ferguson said.

Dr. Ferguson said that hospital employees who were exposed to this virus in London had been quarantined, and that 70 people have been tested so far. Those who caught the disease remained healthy.