Pregnant women are being advised to postpone non-essential trips to Florida because of the Zika virus.
Public Health England updated its travel advice after the first cases of Zika transmitted by mosquitoes on the US mainland appeared in the state.
The health body has said the risk in the state on the south-east coast of the United States is moderate, while many countries in South America, including Olympic host Brazil, is high.
The Zika virus has been associated with a birth defect called microcephaly, which results in children being born with abnormally small heads and brain damage.
More than 1,650 Zika infections have been reported in the US, but four patients in Florida have tested positive for the virus and appear to be the first not linked to travel outside the US mainland.
A total of 53 people have been treated in the UK for the infection.
The updated travel advice reads: "The risk in Florida is considered moderate based on the number and spread of cases and their demonstrated ability to implement effective control measures for similar diseases such as dengue - a virus transmitted by the same mosquito.
"Pregnant women should consider postponing non-essential travel to affected areas until after the pregnancy.
"At present, only a zone of about one square mile in Miami-Dade County is considered at risk of active transmission."