A city worker fumigating the area to control the spread of mosquitoes at a university in Bangkok. Thomson Reuters The US Centers for Disease Control and Prevention has added four countries to its list of places with local transmission of the Zika virus.

The Zika virus, which is mainly transmitted by mosquitoes, has been spreading around the world since 2015. Once infected, only about 20% of people ever show symptoms, which most commonly include fever, rash, joint pain, and red eyes.

The newly identified countries with mosquito-transmitted Zika are:

Angola

Guinea-Bissau

Maldives

Solomon Islands

It's the first time since November in which the CDC has updated its list of countries with local transmission. (The most recent additions were Palau and Montserrat.) Here's the updated list of all the places with the virus.

Zika is most troubling because of its link to birth defects, including microcephaly — a condition in which a child is born with an abnormally small head — in infants whose mothers have had Zika. The virus has also been linked to a neurological condition called Guillain-Barré syndrome.

In November the World Health Organization said Zika was no longer an international public-health emergency. "We are not downgrading the importance of Zika, by placing this as a longer program of work, we are sending the message that Zika is here to stay," Dr. Peter Salama, the executive director of WHO's Health Emergencies Program, said at the time.