A US infectious disease expert said on Monday that she was not sure when the A/H1N1 flu outbreak sweeping across the United States would come to an end.

"The current virus is transmitted easily from person to person. We are seeing a rise in the number of cases in the United States," Joan E. Nichols, director of the Infectious Diseases Biosafety Level 3 Laboratory at the University of Texas Medical Branch (UTMB), told Xinhua in an interview.

"Only time will tell how far, wide the virus will spread or how long it will spread in the United States," she said.

Nichols is one of a few global researchers who are engaged in research to determine the genetic makeup of the H1N1 virus and the biological response to it. Her research mixes aspects of both virology and immunology in order to assess the impact of the virus and/or host factors on the development of clinical disease, generation of immune response and development of immune memory.

"We are nearing the end of our flu season here, but we are seeing spread of the H1N1 virus due to the lack of immunity to this virus in our population," she said.

"At this point you cannot stop the virus from spreading but you can work to limit transmission and exposure of those people most at risk."

The federal government in the form of the Centers for Diseases Control and Prevention (CDC) has urged people to maintain such good hygienic practices as washing hand frequently, covering mouth and nose when coughing and staying home when sick.

"Current measures are to limit but not control spread," Nichols said, adding that there are two ways the virus spreads -- aerosol transmission and small droplets.

As a member of the UTMB Sealy Center for Vaccine Development and a member of UTMB Center for Biodefense and Emerging Infectious Diseases, Nichols is currently developing human respiratory and immune system models using tissue engineering practices to study pathogens such as human, avian and A/H1N1 flu virus, HIV, SARS and others.

On how the virus would spread in the US in the future, Nichols said:" We are going to see the spread of the virus but in smaller groups and spread out over time."

"We will see the infections spread but slower and in a more controlled fashion so that there is not such a simultaneous worldwide outbreak," she added.

Nichols said she could not tell whether the situation could be turning worse, but added that the deaths associated with A/H1N1 in the United States have been like seasonal flu related deaths due to infection and other underlying health problems in the person that died.

To limit the spread of the virus, the expert called on people to follow the CDC guidelines to limit the spread of the virus and control the speed at which it spreads.

"We cannot stop the spread at this point and it is possible that this virus was in our population as a silent infection for a while before we were able to identify it," she said.

So far, there have been 5,123 confirmed and probable cases of A/H1N1 influenza in 47 US states and the District of Columbia. The current death count is five.