The spread of the Zika virus, which is “strongly suspected” to be linked to a condition in which babies are born with abnormally small heads, has been declared as a “public health emergency of international concern” by the World Health Organization. An announcement was made yesterday after an emergency meeting in Geneva. It is a rare move, a sort-of global SOS call—the last time the organization made such a designation was during the 2014 Ebola outbreak in West Africa.

Up to 4 million people in the Americas may be infected by the Zika virus by the end of the year, according to the WHO.

Stephen Morse, a professor of epidemiology at Columbia University’s Mailman School of Public Health, works to understand the factors leading to emerging infectious diseases such Zika. He answered questions about the outbreak in an AMA in Reddit’s Science community.

Here’s what we learned about the virus.

Where did Zika originate?

Which U.S. states might see the virus?

Left map: approximate U.S. distribution of Aedes aegypti. Right map: approximate distribution of Aedes albopictus. See the original image here.

What precautions can people take if they live in the areas at risk?

Is the virus only a critical concern for women who are pregnant?

What happens if a pregnant woman is infected with the virus? How is microcephaly (the birth defect in which a baby’s head is abnormally small) detected?

How long does Zika remain in the body after it is infected?

Can the Zika virus be sexually transmitted?

Can the virus be transmitted through bodily fluids? What happens to the virus once a patient has recovered? What are the symptoms and who should get tested?

Does the virus have anything to do with vaccines?

Should we eradicate the planet of mosquitoes?

Here’s a rundown of the Zika virus from the WHO:

For the full discussion with professor Stephen Morse, see the original Reddit AMA.