The Centre for Health Protection (CHP) is warning pregnant women about the risks of travelling to areas where the mosquito-borne Zika virus is prevalent.

It made the comment after a 16-year boy tested positive for both the Zika and Dengue viruses following a trip to Thailand. The Zika virus can cause birth defects.

The boy, who was in good health, developed a fever, headache and pain behind the eyes last Sunday. He went to St Teresa's Hospital the same day before being admitted to St Paul's on Monday, where his blood specimen tested positive.

The centre said the boy had been to Ko Lanta Island in Thailand between March 28 and April 3 before returning to Hong Kong three days later. While on Ko Lanta Island, the centre said he took part in outdoor activities and he recalled being bitten by mosquitoes.

The CHP said his condition was stable throughout and he is now in United Christian Hospital.