It was the moment US health departments had dreaded. On 24 April a male healthcare worker arrived in Chicago, via London, from Riyadh, Saudi Arabia, and caught a bus to Indiana.
Four days later, suffering from a cough, fever and shortness of breath, he went to the emergency department of the Community Hospital in Munster, a dormitory suburb outside Chicago.
Tests revealed the man, an American citizen living in Riyadh who had flown home to visit family, had Middle East Respiratory Syndrome coronavirus [MERS-CoV or MERS], the first US case since the deadly virus emerged in Saudi Arabia in March 2012.
Initial reports said he became sick after arriving in the United States. But this week, the World Health Organisation said he went down with a fever on April 14, raising the risk for fellow travellers.
The case triggered a "very active investigation" by the Centres for Disease Control.
The patient was isolated in negative-pressure rooms, where air is filtered and not circulated around the hospital.
After going public on May 2, the CDC said that despite the presence of a lethal virus in the US heartland - WHO says it has infected 254 people and killed 93, mostly in the Arabian Peninsula - and no cure, the case "represents a very low risk to the broader, general public".
This week the hospital said the man would probably be discharged to home isolation soon. So far, it looks like he and the US have dodged a bullet.
"We are still not out of the woods," Jason McDonald, from the CDC's National Centre for Immunisations and Respiratory Diseases, told the Herald.
"The quick determination by state health officials that the patient was suffering from MERS set in motion a series of events for which public health officials have been preparing for nearly 18 months."
The patient worked at a Riyadh hospital, but apparently had no direct contact with MERS patients elsewhere in the facility.
Fifty Munster hospital staff - traced by hospital CCTV and radio trackers on their identity badges - were put in home isolation for 14 days, and the patient's family were told to wear masks outdoors.
Most of the people the patient had contact with in the air or on the bus had been contacted, the CDC said.
None has tested positive for MERS, which may not be as contagious as SARS, a coronavirus that infected 8273 people and killed 775 between 2002 and 2003. But the number of MERS cases may be higher.
A virologist with the Australian Infectious Diseases Research Centre (AIDRC) at the University of Queensland, Ian Mackay, says Saudi Arabia "announces the cases publicly but doesn't seem to pass the numbers on to WHO".
He identified 497 cases, including five more from the United Arab Emirates, this week. Of these, 131 have died, a fatality rate of 26 per cent.
The virus, first reported in Saudi Arabia, has surfaced in Yemen, Kuwait, Oman, the UAE, Tunisia, Qatar, Jordan, Germany, France, Italy, Greece, the United Kingdom and the United States.
Last month a Malaysia man who made a pilgrimage to Saudi Arabia died, and a Filipino health worker who had contact with a MERS victim, also a hospital worker, became ill.
Six other Filipinos are also reportedly infected, and officials are working to trace contacts.
Despite contact with patients the fatality rate for healthcare workers is 17 per cent, or 86 cases.
People with kidney disease, diabetes, heart disease, cancer or other serious ailments are most at risk. Significantly, given fears of a pandemic, human-to-human transmission does not seem sustained.
Three-quarters of MER cases are secondary, and only two "possible" tertiary cases are known.
"The virus isn't spreading very well between people," says Mackay. "If someone gets a case they don't tend to pass it on to more than one other person. Humans are looking like a dead-end host at the moment."
But an AIDRC graph shows incidence of the virus is rising, with as many cases this year as in the past two. The virus has a 2 to 14-day incubation period and the number of cases appears to rise from March-April onwards.
This may reflect better surveillance by health care authorities. Or it may be evidence that MERS originates in dromedary camels, as suggested by the journal mBio in February. Camels give birth in the northern spring. Animal-human transmission, most likely to herdsmen but also to people at animal fairs, is via the camel's respiratory secretions.
Researchers from Colombia University's Mailman School of Public Health, King Saud University in Saudi Arabia and EcoHealth Alliance took live, infectious MERS nasal samples from two camels and matched them with the virus in humans. Young camels seem the most infectious.
The scientists discovered camels - which seem immune to the virus - have a wide range of MERS viruses, while humans have a narrow range, one possible reason the disease is fairly uncommon in people.
Using stored camel blood samples, researchers tracked the virus to 1992, although it may have been around longer. Big questions remain. Why did the virus emerge in 2012? What are its transmission routes? Does MERS have a seasonal pattern? Could it mutate to become more transmissible? What risk factors drive outbreaks? How many people have antibodies?
Data is vital. Saudi Arabia, a closed society, has been criticised for not sharing information with researchers. There are unnerving parallels with China, which initially kept quiet about SARS, possibly enabling the virus to get a deadly foothold. Last month, the kingdom replaced its Health Minister and promised greater transparency and disclosure of information.
That is timely, as MERS is the latest killer virus to emerge from a poor, remote area and quickly spread into the wider world.
Climate change, which drives plants and animals to higher latitudes and altitudes, also raises the stakes.
It is a shift seen in mosquito-borne diseases such as malaria and dengue fever.
"There are certainly a lot of hypotheses about the impact of changing weather patterns and seasons on the animals that we get most of these viruses from," says Mackay.
Animals might change the time and place when they give birth, for instance. The camel breakthrough has intensified research on when herdsmen come into contact with camels and how this relates to a later spike of infections.
MERS is not as lethal as the bird flu virus - the 1997 outbreak had a 66 per cent mortality rate - or annual influenza epidemics, which WHO says kill 250,000 to 500,000 worldwide.
And despite fears of a wider MERS outbreak triggered by the annual Hajj pilgrimage - held this year in October - when millions of Muslims visit Saudi Arabia, and reports of more cases from the kingdom (seven this week), there are no signs air travel will be restricted.
But anti-terror border security is now in the front line to detect the most deadly terrorists of all, pathogens that test everyone's defences.