By Alexandra Sifferlin
October 1, 2014

On the afternoon of Sept. 28, U.S. Centers for Disease Control and Prevention (CDC) epidemic intelligence officers—sometimes referred to as disease detectives—received a call from Texas Health Presbyterian Hospital of Dallas concerned about a patient the hospital thought could be at risk for Ebola. This was at least the 90th call the team had received during the Ebola outbreak from hospitals concerned that one of their patients could possibly be infected. But until Tuesday, there had never been a diagnosed case.

CDC director Dr. Tom Frieden said in a press conference on Tuesday that “As long as the outbreak continues in Africa, we need to be on-guard.” The statement rings true since the U.S. now has its first patient, but the CDC has been awaiting the possibility that Ebola would make it to American soil for months.

On Sunday afternoon, an epidemic intelligence service officer took the hospital through a decision-tree of sorts that helps the CDC determine whether the patient is at a real risk for Ebola. Factors that are taken into account are where the patient had traveled in the past and what their symptoms were. Since the patient continued through the CDC’s algorithm with enough red flags, the CDC requested that the patient be isolated and that a blood specimen be sent to the CDC’s level 4 testing lab for confirmation.

MORE: Inside the CDC’s Emergency Operations Center Tackling Ebola

The Dallas patient is the 13th person that the CDC has actually tested for Ebola. “Every morning, Dr. Frieden is updated on all of the individuals that we have looked at and the numerous individuals under investigation,” a CDC spokesperson told TIME. There have been a few false alarms already, including patients in New York City and Miami—all eventually tested negative.

The CDC campus is in Atlanta, Georgia, and since early August, the headquarters has had its Emergency Operations Center on a Level 1 response—the highest possible level for a public health crisis. Just a couple days after the CDC kicked operations into high gear, the World Health Organization (WHO) declared the outbreak in West Africa a global public health emergency. In the Emergency Operations Center, several epidemic intelligence officers sit in rows of long tables tapping away at their computers facing a wall of computer screens that show where Ebola clusters are in West Africa, as well as graphs of the disease’s trajectory. The officers offer aid both domestically and to their colleagues in the field. Many have been traveling in and out of West Africa since the spring.

After the CDC determined that the patient, reportedly a man named Thomas Eric Duncan, was indeed at a very high risk for the disease, the hospital sent blood specimens for testing to both the CDC’s lab as well as a Texas Health Department lab. The specimens arrived at the CDC around 10 a.m. on Tuesday morning, and by Tuesday afternoon, both the CDC and the Texas Health Department had confirmed that the patient was in fact positive for Ebola. “We made sure the hospital spoke with the patient and their family first,” said a CDC spokesperson to TIME.

Once the patient was told they were positive, the CDC quickly informed the public by sending out a confirmation to media late Tuesday afternoon and holding a press conference an hour later. During that time, CDC disease specialists were already deploying to Dallas—landing on Tuesday evening to begin the process of tracking down and monitoring all the people that the patient with Ebola had come in contact with while infectious. It’s a process that will continue until the 21-day incubation period of the disease ends.

Though the Dallas patient is the first patient to have confirmed Ebola, the CDC has long said that an Ebola patient making it to the U.S. was always a possibility. However, due to the quality of health care in the United States, patients are not facing the same dire situations as patients in Liberia, Sierra Leone or Guinea, and Ebola in the states will likely have a much different prognosis.

Contact us at editors@time.com.

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