A sharp increase in infections with a deadly new virus in the Middle East is alarming public health officials around the world. The rising numbers have raised fresh fears that the Middle East respiratory syndrome (MERS) virus has adapted and is becoming more easily transmissible between humans, which could result in global spread. But preliminary research has not shown any evidence of genetic changes, and the government of Saudi Arabia, where most of the new cases occur, says the sudden upswing is mostly the result of more widespread testing.

The MERS virus is related to the SARS virus, which spread around the world in 2003 and killed almost 800 people. Since MERS was discovered in 2012, there has been a steady trickle of cases, the vast majority of them in Saudi Arabia. By late March, the World Health Organization (WHO) had counted 199 cases worldwide, including 84 deaths.

But in the past few weeks, the numbers have shot up. Saudi Arabia is announcing new cases almost every day, most of them in Jeddah, a city in the western part of the country. Yesterday alone, the kingdom reported 24 new cases; another 12 were reported today. (WHO is waiting for official reporting of the cases, but the European Centre for Disease Prevention and Control now says the total is 333 cases, including 107 deaths.) Amid increasing worries in the public and intensifying media coverage, Saudi Arabia's minister of health was replaced on Monday. WHO issued a press release yesterday saying it was "concerned about the rising number of cases." "WHO urges all Member States to remain vigilant and enhance surveillance to detect any early sign that the virus has changed and has attained the possibilities of causing sustained person-to-person transmission," the statement read.

"It is really important to understand now what is going on in Jeddah," says Marion Koopmans of Erasmus MC in Rotterdam, the Netherlands. One possible explanation, she says, is that the lab doing the tests has a contamination problem, leading to false positives.

Saudi Deputy Minister of Health Ziad Memish, an infectious disease specialist, says the explanation is quite simple. Until recently, in accordance with WHO recommendations, only patients with pneumonia who needed intensive care were tested for the MERS virus, he says. But as anxiety about MERS increased, patients with milder symptoms like fever or a cough started demanding to be tested, he says, and doctors in Jeddah have complied. "In the last 2 years we tested 20,000 people," Memish says. "Just in the last weeks, we tested 5000 people." As a result, more mild cases are being picked up, he says, but that does not mean MERS is about to go global.

Nonetheless, studies are under way to test whether the virus has undergone genetic changes. On Friday, virologist Christian Drosten of the University of Bonn in Germany, who is collaborating with Memish, received 31 fresh samples from patients in Jeddah. The scientists isolated MERS virus RNA from all but one of the samples they received, suggesting that contamination isn't the problem.

Drosten has sequenced part of the gene for the so-called spike protein, which sits on the viral surface, from all 30 viruses. They appear to be identical to each other and to several older MERS viruses that have been sequenced. "There seems to be nothing special about this virus, at least in this region of the genome," he says. The researchers are now sequencing the whole genome for some of the viruses and they may still find significant changes, although Drosten doubts that they will.

The variety found between the 30 genomes may also give clues about what is going on. If the viral genomes from different patients are more or less identical, that suggests a new, more successful strain is breaking out; if the genomes are more diverse, the increase is more likely to be an artifact due to more testing. The fragment sequenced so far does not give a clear answer, because a number of different MERS strains are identical in that part of the genome. "But hopefully we will know in a few days," Drosten says.

Meanwhile, Drosten says a seasonal effect may also explain part of the current rise in cases. In breeding facilities across the Middle East, camels, an important reservoir for MERS, have given birth the past 2 months; scientists believe their young may become infected very early on, causing virus levels to explode and increasing the risk for humans to become exposed.

Memish agrees that there seems to be some seasonality in how MERS strikes humans: The first known human cases were from a cluster in Jordan in April 2012, he says, last year in April an outbreak struck four hospitals in the Al Hasa area in Saudi Arabia, and now there is a cluster of cases in Jeddah. "It certainly seems that April and May are bad months for MERS."