The MERS virus. REUTERS/Bernadett Szabo A new spike in cases of a deadly respiratory virus in Saudi Arabia and the United Arab Emirates is prompting new fears of an outbreak when the area's population spikes during the annual Hajj pilgrimage.

The syndrome, called Middle East Respiratory Syndrome (MERS), is caused by a relatively new-to-humans virus that's a close cousin of SARS, a virus that infected thousands of people worldwide in 2002-2004.

Millions of Muslims from around the world make pilgrimages to Mecca, and reports yesterday of the first confirmed case in the holy city raised concerns that MERS could spin out from the Middle East and spread more widely as people move into and out of the city.

Saudi Arabia has already sacked its health minister, a move that was widely interpreted as a response to his mismanagement of the growing outbreak.

Here's what you need to know about MERS, which currently has no vaccine and no cure.

1. The recent spike in cases is alarming.

The first confirmed cases of MERS in humans — scientists believe it jumped from camels — were reported in 2012. In 2013 — and it seems again this year — cases of the disease peaked in March/April. But this year's outbreak seems more severe.

"It took two years since those first cases for the global tally to hit the 200 mark. That happened in late March [2014]," Helen Branswell explains. "Now the combined total announced by the WHO (World Health Organization) and affected countries is over 360 cases." There have been 80 new cases in Saudi Arabia in the last week alone, Branswell reports.

2. We don't know exactly how many people are infected.

Branswell came to the 360 number by adding up all officially reported cases, but the number of cases confirmed by the World Health Organization is still just 254, with 50 in the last week. There is some lag in the WHO's numbers given their process to confirm diagnoses that have been reported by individual countries. Saudi Arabia, for example, reports 297 cases in their country alone.

Furthermore, these are just the severe cases — some people may be asymptomatic, and some symptoms may be so mild that they are not reported. Still, even the most conservative estimates show a significant jump in the number of cases in the past month and the past week.

3. MERS is considered a more deadly, but less contagious cousin, of SARS.

Like SARS, MERS is caused by a coronavirus — a class of viruses that cause respiratory illnesses of varying severity and, under a microscope, look like they have spiky crowns. SARS killed fewer than 10% of the estimated 8,000 people infected; MERS has killed about 30% of those with confirmed infections, although those whose health is already compromised seem more likely to be infected.

MERS starts out a bit like a cold, but can escalate into pneumonia, organ failure, and death. The onset of MERS is faster than SARS and generally requires closer contact to be transmitted. In the past, MERS has appeared to be easier than SARS to contain and fight off (for healthy people).

"While MERS is a bad actor, it is no SARS and most likely will not cause a pandemic," writes microbiologist Jason Tetro at Popular Science.

4. MERS might be more contagious now than in previous years.

Scientists suspect (but don't yet know for sure) that MERS cases in humans tend to come from contact with camels. But the WHO reports that as many as 75% of the recent cases have been transmitted from human to human, generally in hospital settings.

"There's a major change occurring that cannot just be attributed to better case detection," Dr. Michael Osterholm, Center for Infectious Disease Research and Policy at the University of Minnesota, told NPR. "Something's happening."

This could be because the virus has changed into something more contagious, or because methods of infection control have been inadequate.

Fortunately, it seems that while infected person A can spread MERS to person B, person B is highly unlikely to then be contagious enough to spread it further, say, to person C. There have only been two such cases of this so-called "tertiary" transmission so far.

Still, because the virus seems to be spreading between humans more than it did in the past, the WHO is on alert: "Urgent investigations are required to better understand the transmission pattern of this virus."

5. MERS will continue to spread beyond the Middle East.

The recent spike in cases is primarily in Saudi Arabia (especially in Jeddah) and the UAE, but — as with other recent outbreaks — it is expected to spread beyond those countries. "The spike in cases, especially among health-care workers, could be a signal that the virus has reached a tipping point and could be ready to spread out of the region," NPR reports.

Since 2012, MERS has been confirmed in Jordan, Kuwait, Oman, Qatar, Saudi Arabia, the United Arab Emirates, France, Germany, Greece, Italy, the United Kingdom, Tunisia, Malaysia and the Philippines.

Cases that have been "exported" from the current outbreak centered in Saudi Arabia have not spread beyond the infected travelers, but MERS will probably crop up in more far-flung places soon.

"It is very likely that cases will continue to be exported to other countries," WHO predicts. "Whether these cases will [spread further] will depend of the capacity of the receiving country to rapidly detect, diagnose and implement appropriate infection prevention and control measures."