Three Oregonians are among 31 cases of Zika virus reported in the United States, but experts say the risk of a large outbreak here is slight.

All of the cases are travel-related, according to the Centers for Disease Control and Prevention. The three people in Oregon traveled to Polynesia, one in 2014 and two in 2015, said Dr. Richard Leman, Oregon public health physician. He does not know whether any of them were pregnant but all recovered, he said.

South of the United States the virus is exploding, with cases from Mexico to Paraguay.

Brazil has been especially hard hit. Health officials fear it may be linked to an epidemic of babies born with shunted heads and an increase of a neurological disorder. There's no treatment or vaccine.

"Until now there's not been a lot of research," said Dr. Mark Slifka, a senior scientist at Oregon Health & Science University. His lab studies viruses similar to Zika, called flaviviruses. They include West Nile, dengue and yellow fever.

The virus first popped up in a monkey in the Zika Forest in Uganda in the late 1940s, thus the name. Before this latest outbreak, people have become infected in Africa, Southeast Asia and the Pacific Islands.

Though first identified in a monkey, the Zika virus also uses mosquitoes as a host. Slifka said if the virus travels from mosquitoes to humans and monkeys and then back again, it's unlikely to gain a strong foothold in the United States.

"You have monkeys in South America but we don't have a monkey population that could keep that cycle going," he said.

West Nile virus has thrived in the United States, including in Oregon, because it lives in birds, giving it a wide population of hosts. Two types of mosquitoes carry the West Nile virus. Only one of them, Aedes aegypti, appears to also be able to carry Zika.

This mosquito thrives in tropical climates. There aren't any in Oregon, Leman said. Though they do exist in the South, the United States has better mosquito control than South America, Slifka said.

"We have window screens and air conditioning and we're able to spray urban areas," Slifka said.

Still, pregnant women need to take care if they plan on traveling to any affected countries. Public health officials advise against it. But if travel is necessary, they should cover up and use mosquito spray. The virus appears to pose a particular threat during the first trimester.

So far, it looks unlikely that people can transmit the virus to others though there was one case of a person infecting their spouse, Slifka said.

"Right now it's mainly spread by mosquitoes," he said. "We have to wait until more is known."

What is known is that these viruses are tiny. They only have 10 genes, Slifka said. They're much less complex than the malaria parasite, for example, which goes through different cycles as it passes from mosquitos to humans and back again.

Zika is also harmless to most people. Only about one in five get symptoms, which are similar dengue fever: rash, joint pain and fever.

But there has been an increase of a rare neurological disorder that physicians in South America fear may be related to Zika. Guillain-Barre syndrome causes a person's immune system to attack nerve cells which can cause muscle weakness and even paralysis. The birth defect, microcephaly, causing the brain to not fully develop in the womb, has mainly affected women in Brazil.

With a world spotlight on Zika, Slifka expects to see a vaccine, just as there was with Ebola. An OHSU start-up, Najit Technologies, Inc., plans a vaccine research program. It has already developed vaccines for West Nile virus, yellow fever and dengue.

-- Lynne Terry