The death toll from the Middle East Respiratory Syndrome coronavirus has risen to 53, according to the World Health Organization (WHO), and on Friday, the CDC confirmed the first case in the United States, a man from Qatar diagnosed with MERS.

After 11 months of spread to a variety of countries, some of the earliest cases in the world appear to have originated in Saudi Arabia. Here’s how the virus seems to have spread:

First, patients reported a severe cough.

We know this is classic coronavirus signature, but that doesn’t account for all of the reported cases. In fact, the level of severity may have shifted by location. Here’s a quick look at which cases were hard cases.

In the top two graphs you can see that patients reported a combination of the following symptoms: fever, cough, shortness of breath, and runny nose. Since 2010, every case with either a cough or fever was accompanied by a “slightly elevated” respiratory tract secretions, as a systematic review of cases did.

In the bottom graph you can see that the case history for those with the lung infection has shifted the disease’s geographical boundaries. Cases with fewer respiratory symptoms, and of a lesser severity, have spread more widely outside of the Middle East.

That seems to map out the spectrum of what individuals who have had MERS in the past have experienced: shock, or even worse. The number of Saudis in other Middle Eastern countries with respiratory symptoms has jumped, for example. People from other parts of the world who have had mild respiratory symptoms with their MERS have also spread. What is unclear is whether there are other areas where coronavirus is becoming prevalent, such as the Pacific or the Asia-Pacific.

After 11 months of international spread, the case from Qatar was confirmed on April 8. It means that the virus may have entered the United States a little earlier than predicted. Until it turns up in some of these detailed case reports, however, the exact route of introduction and the exact number of Americans possibly infected is unknown.

Read more on this case in The Washington Post