The number of people diagnosed with Ebola in the United State rose to four Thursday, when a doctor in New York City tested positive for the disease. Five others infected in the West Africa outbreak have been treated here. A still far wider net has been cast to keep an eye on dozens of others for symptoms.

Earlier this week, U.S. officials announced new requirements for travelers from affected countries aimed at finding people who enter the U.S. before they start showing signs of Ebola. "Someone infected with Ebola may not show symptoms for up to 21 days," Centers for Disease Control and Prevention director Dr. Thomas Frieden said. With the new measures, travelers from Liberia, Sierra Leone, and Guinea must check in with health authorities daily for three weeks, even if they show no symptoms.

In New York City, Dr. Craig Spencer tested positive for the disease, hours after he came down with a 100.3-degree fever and gastrointestinal symptoms. He had recently worked with Doctors Without Borders in one of the countries affected by the Ebola outbreak.

Here’s a look at the confirmed cases and the people being watched most closely.

Patients evacuated to the United States

Dr. Kent Brantly: Infected in Liberia while working for a Christian aid organization. Flown to Emory University Hospital in Atlanta, one of four U.S. facilities with a specially equipped isolation unit for Ebola patients, on Aug. 2. Recovered after receiving an experimental drug and was released Aug. 21. He has since donated his blood to several other patients in the hope transfusions would aid in treatment.

Nancy Writebol: Infected in Liberia while working for a Christian mission. Flown to Emory on Aug. 5. Recovered after receiving an experimental drug and was released Aug. 19.

Dr. Rick Sacra: Infected in Liberia while working for a Christian mission. Flown to the Nebraska Medical Center in Omaha, another specially equipped facility, on Sept. 5. Recovered after receiving an experimental drug and a blood transfusion from Brantly and was released Sept. 25. Went to a Boston-area hospital as a precaution in early October with a suspected case of pneumonia.

Unidentified patient: An American doctor with the World Health Organization who was infected in Sierra Leone. Flown to Emory University Hospital on Sept. 9 and spent six weeks recovering in the hospital's special biocontainment unit. The doctor asked not to be named, but said in a statement released by Emory on Oct. 20 after he went home, "I want the public to know that although Ebola is a serious, complex disease, it is possible to recover and return to a healthy life."

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Ashoka Mukpo: Cameraman diagnosed with Ebola while working in Liberia as a freelancer for NBC News. Flown to Nebraska Medical Center on Oct. 5. Was cleared of the virus on Oct. 21 after receiving care that has become standard in the U.S.: testing and replacing electrolytes, which get flushed from the body during the intense vomiting and diarrhea that are hallmarks of mid-stages of Ebola. “After enduring weeks where it was unclear whether I would survive, I’m walking out of the hospital on my own power, free from Ebola. This blessing is in no small measure a result of the world class care I received at the Nebraska Medical Center,” Mukpo said in a statement.

Patients diagnosed in the United States

Dr. Craig Spencer: tested positive for the disease, hours after he came down with a 103-degree fever and gastrointestinal symptoms. He was taken to Bellevue Hospital.

Thomas Eric Duncan: Liberian national who flew to Dallas on Sept. 19 and 20 to see family. Went to Texas Health Presbyterian Hospital with symptoms on Sept. 25 but was sent home — a mistake, the hospital acknowledged later. Went back on Sept. 28 and was confirmed positive for Ebola on Sept. 30. Died Oct. 8.

Nina Pham: Nurse at Texas Health Presbyterian who treated Duncan. Tested positive on Oct. 11. Health officials have said it is unclear how she was infected. Was flown to a National Institutes of Health facility in Bethesda, Maryland, specially equipped for Ebola, on Oct. 16. Was upgraded from fair to good condition on Oct. 21. Meanwhile, her dog Bentley, a Cavalier King Charles Spaniel who was put under a 21-day quarantine period, tested negative for the virus, Dallas officials said on Oct. 22.

Amber Vinson: Nurse at Texas Health Presbyterian who treated Duncan. Flew to Cleveland on Oct. 10 to plan her wedding, and back to Dallas on Oct. 13, after calling the CDC to report a slight fever and being told she was OK to fly. Tested positive Oct. 15. Flown to Emory the same day. Health officials have said it is unclear how she was infected. On Oct. 22, Vinson's blood tests came back negative for the virus, although she has not been declared Ebola-free and will not leave Emory yet.

Quarantined

— Four people who had contact with Duncan, including his fiancée, Louise Troh, were ordered quarantined for 21 days by authorities in Texas. They finished the quarantine with no signs of the virus, Troh's pastor said on Oct. 20.

— Three people in Ohio who had contact with Vinson, the nurse, are under quarantine, health officials there say.

— Six West Africans who recently arrived in the U.S. are under quarantine for 21 days in Connecticut, authorities say. They have not specified what country the family came from.

— Dr. Nancy Snyderman, the chief medical editor for NBC News, has said that she and her newsgathering team, which worked with Mukpo in Liberia, were on mandatory quarantine for 21 days. Their quarantine period ended Oct. 22 with no evidence of the virus.

Asked not to travel

— 75 health care workers who treated Duncan at the Dallas hospital, and who are being monitored for Ebola symptoms, have been asked by authorities to agree not to take public transportation or visit public places.

Being monitored for symptoms

— 108 people in Texas who are thought to have had contact with Duncan are being monitored for symptoms themselves. Another 66 have passed through the 21-day monitoring period without showing symptoms.

— Several people who were on each flight that Vinson took, the flight to Cleveland and the return to Dallas, are being monitored.

— A total of 163 people in Ohio are being monitored because of contact or potential contact with Vinson, the Department of Health there said on Oct. 23.