Texas health officials are reporting five cases of chikungunya, a virus that has been tearing through the Caribbean.
Travelers have acquired the virus while visiting areas where the virus is most common.
Imported cases mean there is a potential for chikungunya to spread in Texas because the Aedes mosquitoes that transmit it are present in the state. The mosquitoes have distinctive black and white stripes.
Health officials urged residents to prevent mosquito bites, but said there was no cause for alarm.
"There is no broad risk to the health of the general public," said Dr. Celeste Philip, a public health official with the Department of Health.
Federal officials noted it's an unfortunate milestone in the spread of a painful infectious disease that has raced across the Caribbean this year and is apparently now taking root in the United States.
"The arrival of chikungunya virus, first in the tropical Americas and now in the United States, underscores the risks posed by this and other exotic pathogens," said Roger Nasci of the Centers for Disease Control and Prevention, in a prepared statement.
Chikungunya virus is rarely fatal. Infected people typically suffer fever, severe joint pain and swelling, muscle aches, headaches, or rash. Patients usually recover in about a week, although some people suffer long-term joint pain. There is no vaccine and no specific treatment for it.
This virus is not spread person to person, but rather by the bite of certain mosquitoes. That's why health officials believe the virus is spreading here -- two people who contracted the virus in Florida had not recently left the country.
More than 230 chikungunya cases have been reported in Americans this year, but all the others were travelers believed to have been infected elsewhere.
Now that chikungunya is in the United States, CDC officials think it will behave like dengue virus, with imported cases causing occasional local transmissions but not widespread outbreaks.
Chikungunya was first identified in 1952 in Tanzania. It first appeared in the Americas late last year, on a Caribbean island. By July 11, more than 355,000 suspected and confirmed cases were being reported in the Americas.
Officials have been expecting it to land in the United States, noting that two types of mosquitoes that can carry the virus live here.
Earlier this week, the New England Journal of Medicine published an editorial on the looming chikungunya threat by two National institutes of Health infectious diseases experts -- Dr. David Morens and Dr. Anthony Fauci.
They noted several chikungunya vaccines are being developed. But even if they prove effective, they would be years away from becoming available.
Mosquito control and avoidance are the best current options, they said.