A patient diagnosed with the Zika virus in Texas became infected after having sexual contact with another sufferer, health experts said today.

It marks the first case of sexual transmission of the virus in the US, the Centers for Disease Control and Prevention confirmed. Dallas County health officials said the patient’s partner fell ill after visiting a country where Zika is rife.

The World Health Organization has declared the Zika virus a global public health emergency. The infection may be linked to thousands of babies being born with underdeveloped brains.