An Aedes aegypti mosquito is photographed through a microscope at the Fiocruz institute in Recife, Pernambuco state, Brazil. (Felipe Dana/Associated Press)

A reported 18 people in Virginia have known cases of the Zika virus, according to the latest numbers from the state’s Health Department.

The state does weekly reporting of the number of Zika cases, officials said. That latest count is as of Thursday.

In Maryland, 17 people reportedly have the virus, and four in the District have it, according to figures the Centers for Disease Control and Prevention released Wednesday. Nationally, there are 591 cases, the CDC reports.

Those who have the virus were infected while traveling outside the United States. But officials warn that that could change as the number of bugs increases in warmer weather.

Everything you ever wanted to know about the Zika virus and its spread across North and South America. (Daron Taylor,Claritza Jimenez/The Washington Post)

The Zika virus has also been found in the Aedes albopictus, a species of mosquito that is known as the “Asian tiger” and lives as far north as New England. The discovery means the number of states that are at risk for transmission of the Zika virus has expanded.

Authorities have said the South is particularly susceptible to the spread of Zika, because of the warm, humid climate there. The White House has asked Congress for about $1.9 billion to help local and state officials prepare for an outbreak of Zika this summer. The money would go to mosquito-control efforts such as spraying and larvacide.

Alison Ansher, health director for the state’s health department in the Prince William County district, said part of the challenge in tracking the Zika virus is that a large percentage of people who become infected don’t immediately display symptoms.

If someone travels overseas and becomes infected, for example, the person may not know it is in their blood. If they go out and are bitten by a mosquito, then that mosquito is infected, and it may bite someone else.

Symptoms of the Zika virus include rash, fever, joint pain and conjunctivitis. Typically, the symptoms are mild, and many people who have the virus may not realize they are infected, health experts said.

Pregnant women face the biggest risk if they become infected. The virus can cause a birth defect called microcephaly, a condition that causes a baby’s head to be much smaller than normal. Infected babies also can have smaller brains that have not developed correctly.

Several Latin American countries and Brazil, which has been considered the epicenter of the virus, have been hit hard by the Zika outbreak.

Health officials warn that area residents should take precautions such as wearing long sleeves and pants to provide the best protection from mosquitoes. Others tips include making sure window screens are in place and work properly and following the instructions on labels for insect repellent. They also warn that items even as small as a cap filled with water can provide breeding grounds for mosquitoes and should be cleared.