DENVER -- There have been no confirmed cases of swine flu in Colorado, but some doctors believe it is already here.

Dr. Don Lefkowits of Rose Medical Center said he is awaiting tests of three patients who showed up at the hospital this week.

Two of them had just returned from Mexico and he said he is fairly certain they have swine flu.

The hospital has set up a reception desk to greet patients and visitors.

Anyone showing signs of the flu is given a mask and when appropriate patients are given anti-viral medications.

He told 7NEWS it was pretty much unavoidable that the disease would end up in Colorado.

He added, "It's too early to know if the outbreak has been relatively mild. The jury is out on how severe it will be."

Vaccine researchers for the Denver VA and University of Colorado Denver School of Medicine are closely monitoring the swine flu outbreak.

"Something that may be more common with this new virus is diarrhea and vomiting," explained Dr. Ed Janoff. He said other symptoms are similar to what is commonly seen in seasonal influenza cases.

Janoff encouraged anyone presenting signs of swine flu to stay home and seek medical treatment if symptoms do not diminish within a few days.

"Most of these cases have been mild. If you're very sick you'll know it," Janoff said. "The vast majority of cases will go away by themselves."

He added that there are plenty of unknowns with the virus. Scientists across the country are developing a vaccine that could be put on a fast track by the Food and Drug Administration if conditions warrant, Janoff said.

More than 1,600 cases of swine flu have been reported in Mexico, and the suspected death toll there is nearly 150.

The U.S. stepped up checks of people entering the country by air, land and sea, but Denver International Airport officials said they were taking only normal precautions.

Transportation Security Administration workers at Denver International were allowed to wear gloves and masks but weren't required to, airport spokesman Jeff Green said.

Green said the airport had its normal contingent of paramedics on duty but no extra staff. If an incoming flight crew noticed passengers with symptoms, a paramedic would board the plane to check, but that's normal procedure, he said.

Denver Public Schools reminded teachers and students to wash their hands and take other normal precautions but didn't plan any extra scrubbing.

The Mesa County School District in western Colorado said it planned no extra cleaning and that the county health department was handling advisories on prevention.

Jefferson County Public Schools, the state's largest district, said it was taking normal precautions but nothing extra.

The state health department opened its emergency operations center and set up a phone line (877-462-2911 ) to answer questions.

"We're preparing and being as aggressive as we possibly can," said Chris Lindley, the state health department's director of emergency preparedness.

7NEWS reporter Lane Lyon contributed to this report.