DALLAS (KABC) -- An unnamed patient at a Dallas hospital has been isolated and is in intensive care after being diagnosed with the Ebola virus, the U.S. Center for Disease Control and Prevention announced Tuesday. The case marks the first in the nation.The Texas Health Presbyterian Hospital of Dallas said the patient had traveled through Liberia and left the country with no symptoms on Sept. 19 to visit family in the U.S. He developed symptoms four days after his return and arrived at the Dallas hospital on Sunday.CDC Director Tom Frieden said a "handful" of people, including family members, who may have been exposed are being monitored. The Texas Health Department is also helping the CDC trace others who may have been exposed, Frieden said.Frieden added that President Barack Obama was briefed about the case early Tuesday.Ebola symptoms can include fever, muscle pain, vomiting and bleeding, and can appear as long as 21 days after exposure to the virus. It can only be spread through blood and other bodily fluids.The virus has killed 2,917 people in West Africa and infected at least 3,000 more since the outbreak began in March, according to the World Health Organization. International and U.S. health officials have warned the outbreak could infect 1.4 million people in West Africa by January if more is not done to stop the disease.Health experts don't expect infection in the U.S. to spread."Here in the United States, we know how to prevent the spread of these infections. People need to really understand that this is being taken care of. It is in a hospital. We're not concerned about this becoming a national problem," said Dr. Anne Rimoin, an associate professor at UCLA.Dr. Jeffrey Gunzenhauser, interim health officer for Los Angeles County Health Department, said Los Angeles is prepared in case the virus were to spread."We've been talking with hospitals. We've been increasing our lab capacity. We've talked with our staff about identifying contacts and how we would do that," Gunzenhauser said.