WASHINGTON (AP) — Texas health officials have confined four people to their home, under guard, after they had close contact with an Ebola patient hospitalized in Dallas, as disease detectives work to make sure the deadly virus doesn’t spread in the U.S. Five things to know about containing the virus:

1. WHY ORDER PEOPLE TO STAY HOME

Texas officials say those four ordered to stay put and have no visitors weren’t complying with requests to voluntarily stay home for 21 days, Ebola’s incubation period. Officials have reached out to as many as 100 people to determine if they had any contact with the patient, Thomas Eric Duncan, or someone close to him. They are casting a wide net so as not to miss anyone. But so far officials consider the at-risk group about 12 to 18 people who had direct contact with the man. Some who have been asked to stay home during the incubation period have complied voluntarily.

2. HOW ARE PEOPLE MONITORED

Every day for 21 days, those people are checked for a fever or other symptoms. Depending on the circumstances, health workers might visit them to take their temperature or trust them to phone in a report. Symptoms can begin anywhere from two to 21 days after exposure, but eight to 10 days is the usual range.

3. WHAT IS SUPPOSED TO HAPPEN WHEN SOMEONE WITH POSSIBLE EBOLA SYMPTOMS ARRIVES AT THE HOSPITAL

The Centers for Disease Control and Prevention has given hospitals a checklist to make it easy:

• Do they have a fever of 101.5 degrees?
• Do they have other Ebola-like symptoms, such as flu-like body aches, vomiting or diarrhea? If so, they’re told to ask whether the person traveled to an Ebola-infected country during the past 21 days. Duncan revealed in a Dallas emergency room last week that he was visiting from Liberia, but the hospital has acknowledged it didn’t pass that information to doctors who ultimately diagnosed a low-risk infection and released him. Duncan returned by ambulance on Sunday and was isolated; tests confirmed Ebola on Tuesday. The hospital now says it has altered its patient records computer system so that doctors will automatically have access to a patient’s travel history and, specifically, to any Ebola-endemic regions of Africa.

4. WHAT ARE HOSPITALS ELSEWHERE DOING

The CDC is repeating its instructions to health workers, and many hospitals say they’re spreading the word to staff again after the Dallas case. Regions Hospital in St. Paul, Minnesota, says it got a practice run a few weeks ago when the emergency room asked someone with possible symptoms about recent travel, learned he’d been to Liberia and immediately isolated him until doctors determined he didn’t have Ebola. In rural Coldwater, Ohio, a hospital even posted signs at main entrances telling patients to tell staff members if they had traveled to West Africa recently.

5. DOES SCREENING AIR TRAVELERS HELP

The CDC said the Dallas patient had no fever or other symptoms while traveling and wasn’t contagious at that point. But Liberian authorities have alleged he lied on a form about not having any contact with an infected person, although it’s not clear if he knew the person had Ebola.

A spokesman for the U.N. secretary-general said Thursday that air travel to the outbreak zone should continue, because further isolating those countries — where much air travel already has ceased — would worsen their political and economic situations and make it even harder to deliver aid.

According to the CDC, between January and July, there were between 750 and 1,750 travelers to the U.S. per week from Guinea, Liberia and Sierra Leone, a number believed to have dwindled as the outbreak worsened. That represents about one-tenth of one percent of the two million travelers who come into the U.S. weekly.

While screening travelers before they leave is important, people don’t always realize if they’ve been exposed to Ebola, said CDC Director Dr. Tom Frieden. “We can’t make the risk zero until the outbreak is controlled in West Africa,” he said.