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This map shows the latest spread of reported cases of coronavirus

The number of reported coronavirus cases continued to grow, with more than 2,600 people dead and some 79,000 people around the world sickened, the vast majority in mainland China, researchers at Johns Hopkins said Monday.

But that's just the number of reported and confirmed cases from an outbreak that has posed a series of challenges for researchers trying to collect reliable data on the spread and severity of the new disease.

"There is no way to get everything," said Dr. Este Geraghty, chief medical officer at ESRI, a mapping software company that is working with researchers at Johns Hopkins University to map the outbreak. "We're always going to have things missing in the data."

Some researchers believe the actual number of coronavirus cases may be much higher that the reported data, a concern that has been highlighted by various changes in reporting methods used by Chinese officials.

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About fifty cases have been confirmed in the U.S., including in Arizona, Washington state, Massachusetts, Illinois, Wisconsin, Texas and California. Many of these cases are former passengers of the Diamond Princess cruise ship who are now in quarantine or hospitals in the United States.

Dozens of airlines around the world have announced cuts or cancellations in service to Chinese destinations, citing safety concerns or a drop in passenger demand.

Coronaviruses are a large family of viruses that usually infect animals but can sometimes evolve and spread to humans. Symptoms in humans include fever, coughing and shortness of breath, which can progress to pneumonia.

Physicians recommended basic hygiene, including washing hands with soap and water and avoiding touching eyes, nose or mouth.

The previously unknown, flu-like coronavirus strain — now known as COVID-19 — is believed to have emerged from an animal market in Wuhan, a city of 11 million in Hubei. A group of scientists said last month that snakes, particularly the Chinese krait and the Chinese cobra, may have been the source of the virus. But officials from the World Health Organization cast doubt on that theory, saying there was "no conclusive evidence."

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The challenge of data collection in any outbreak starts from the moment someone contracts the targeted illness. Some of those infected may be reluctant or unable to go to a hospital or clinic; those cases will go unreported until they do so. Others may have been infected with the disease and show no symptoms — but may still be able to transmit the virus to others.

One reason for changes in reporting new cases: a shortage of test kits in some areas means that health workers have to resort to other methods of confirming that an illness was caused by the coronavirus.

"A number of countries are just beginning to get testing capability, said Dr. Tom Inglesby, director of the Center for Health Security at the Bloomberg School in a recent panel discussion. "So in some places in the world they would not have had a chance to know whether there was disease in that country yet."

When a patient presents at a hospital or clinic with symptoms, each diagnosed case is entered into a running tally from that location and then sent to a central reporting agency, according to Dr. Caitlin Rivers, a senior scholar at the Johns Hopkins Center for Health Security. But that data may pass through several layers of government agencies; not all countries use the same system for collecting national statistics.

The result may be a difference in the time it takes for those cases to show up in the provincial or national totals.

"When people get sick especially in the context of a new outbreak, it takes a while to move them through the detection pipeline and the diagnostic pipeline," she said. "So just because we saw a big influx in cases today, it doesn't mean those people got sick today, but they probably got sick recently."

That's another reason some researchers believe the number of coronavirus cases may be heavily underreported. Some estimates based on computer models suggest there may be five times as many cases as have been reported, according to Inglesby.

— Reporting by Berkeley Lovelace Jr. John Schoen; data visualizations by John Schoen. Reuters contributed.

Correction: In an earlier version of the map on this, the data for cases of deaths and recoveries were inverted. The map has been corrected.