Federal and state health officials said Saturday that they are continuing to monitor the conditions of 49 people in the Dallas area who may have had contact with Thomas Eric Duncan, the first person diagnosed with the Ebola infection in the United States, but none has shown signs of the disease.

The group includes nine people who almost certainly came in contact with Duncan — four in the apartment where he was staying — and about 40 others whom officials cannot rule out, said Tom Frieden, director of the U.S. Centers for Disease Control and Prevention. Because most people begin to show signs of infection within eight to 10 days, and Duncan became contagious Sept. 24, the next few days are a critical period in determining whether anyone else has the disease, he said. But he noted that the incubation period can extend for 21 days.

Duncan's condition worsened from serious to critical Saturday afternoon, according to Texas Health Presbyterian Hospital in Dallas. He is isolated and in intensive care.

Meanwhile, the District of Columbia Department of Health announced Saturday that a person admitted to Howard University Hospital on Friday does not have Ebola. The patient is the second in the past two days to test negative for the virus after showing up at a Washington area hospital with symptoms of the disease. A patient at Shady Gove Adventist Hospital in Rockville, Md., turned out to have malaria, not Ebola, officials said Friday night.

Saturday afternoon, the CDC investigated a passenger on a flight that had landed at Newark Airport. The passenger, who was believed to be from Liberia, according to reports in New York media, had vomited during the flight, said a spokeswoman for the Port Authority of New York and New Jersey, which oversees the airport. Vomiting can be a symptom of Ebola infection, but also of other diseases common to West Africa, such as malaria.

The spokeswoman, Erica Dumas, said CDC and Port Authority workers met United Flight 998 from Brussels, which landed at 12:15 p.m., and removed a man and his daughter. The rest of the passengers were released at 1:50 p.m. The man and his daughter were taken to University Hospital in Newark.

University Hospital said in a statement on its Facebook page Saturday evening that "the symptoms of one individual were found to be consistent with another, minor treatable condition unrelated to Ebola." The daughter was asymptomatic, the hospital said. Both were to be released but will be monitored, the statement said.

The 49 people who will be monitored for 21 days in Dallas include some who later rode in the ambulance that took Duncan to the hospital Sept. 28, when he became violently ill with Ebola. Frieden said officials couldn't be entirely sure that they did not come in contact with the virus and will track their conditions.

The nine highest-risk patients include an unspecified number of health-care workers who came in contact with Duncan while he had symptoms of the lethal hemorrhagic disease but before he was isolated at the hospital. Another four were living in the Dallas apartment where the Liberian man was staying. The have been moved to a private home in another location donated anonymously.

In all, CDC and local authorities have assessed 114 people to determine whether they had contact with Duncan. Frieden stressed that the numbers in cases where such "contact tracing" must be conducted frequently change, as people remember new contacts or officials discover them.

He said the effort shows that the public health system knows how to contain Ebola in the United States, using methods that public health workers employ every day against tuberculosis, measles and other communicable diseases.

"The way to stop Ebola in its tracks is contact tracing and followup," Frieden said.

Frieden and David Lakey, commissioner of the Texas Department of State Health Services, offered no clearer explanation Saturday for the system's most notable mistake: When Duncan showed up at Texas Presbyterian with symptoms of Ebola on Sept. 25 and reported that he had been in Africa, health workers there sent him home instead of isolating him. At first that was blamed on a failure of the medical records system that did not send that information to the entire medical team. But the hospital retracted that explanation late Friday night without elaborating on how the mistake was made.

Frieden said that news of the Dallas patient has created an expected surge in reports of Ebola symptoms around the country, with "well over 100 inquiries" about possible cases. He said more than a dozen labs are now able to test blood for the virus, allowing for a more rapid response to any suspected patients.

This post has been updated.

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