It weighs just 2.5 milligrams and travels about one mile per hour, but this little critter continues to cause mayhem — most recently with its impact on unborn children.

“A mosquito is an ideal environment for viruses,” said UCHealth’s Dr. David Beckham, an associate professor of medicine in the Division of Infectious Diseases at the University of Colorado School of Medicine.

There are about 2,700 species of mosquitoes — 176 in the United States — each having a unique biology that affects their potential for catching, carrying and transmitting infections.

“Mosquitoes develop the infection in their gut, and it gets into their salivary glands, where they can carry it for a good period of time,” Beckham said.

Two types of mosquitoes, A. aegypti and A. albopictus, have recently been responsible for infecting thousands of people in South and Central America, Mexico, Puerto Rico and parts of the Caribbean with the Zika virus. This virus has resulted in an alarming number of children in those regions being born with the serious neurological condition called microcephaly.

“Why just those two types of mosquitoes? We don’t really know, but we think that the virus has evolved to survive in those mosquitoes because of their complex biology that affects their reaction to the virus,” said Dr. Daniel Pastula, neurologist, medical epidemiologist and arboviral disease expert with UCHealth.

A virus’s host is an important component in the spread of the disease.

Zika falls in the same virus family as West Nile and dengue viruses, but unlike its other mosquito-born flaviviruses family members it can be transferred from human to human by vertical transmission to a fetus and sexually from an infected human male through his semen.

“What we have found is that men can have prolonged secretion of active virus in the semen longer than just when they are sick,” Beckham said.

Like dengue and Zika virus, chikungunya virus, though not in the same family, can spread human to mosquito to human. However, humans are a dead-end host for West Nile virus, meaning they can’t spread the virus except through blood transfusion or organ transplant. Currently, it’s unknown whether Zika can be transferred through transfusion or organ transplants but researchers suspect it’s possible.

Different mosquitoes carry the different viruses. While Zika, dengue and chikungunya are carried by A. aegypti and A. albopictus, which are found in more tropical climates and can’t overwinter in colder climates, West Nile is carried by the Culex mosquito, which does winter and is found in Colorado. These mosquitoes are most prevalent from July through September, and people should take similar actions to avoid mosquito bites as they would in Zika prevention.

A virus takes advantage of his host, Beckham explained. For example, the A. aegypti mosquito is a “hyper spreader” of Zika virus. Dubbed the “sipper,” the mosquito doesn’t take full meals from its host, but rather takes a sip and moves on to the next person. It thrives in urban crowded environments and adapts very well indoors. “You combine those features, it makes this particular mosquito a robust host for these viruses,” he said.

Since early July, there have been six reported cases of locally acquired Zika infections in Wynwood, a neighborhood in Miami, Fla., according to the Centers for Disease Control and Prevention. It’s the first reported transmissions of the virus in the U.S. as of Aug. 3, 2016.

Prevention is key: Until more is known about the Zika virus and its effect, people need to follow CDC guidelines and do their best to prevent getting and/or spreading Zika virus, Dr. David Beckham said. Because of the active travel of our society, people may spread the virus more easily into areas of the U.S. and other non-Zika infected regions.

Florida health officials are working to contain the infection through mosquito control programs that includes aerial insecticide treatments.

The A. albopictus mosquito that also carries Zika virus likes to live in forested and heavy brush areas, such as parks, and it enjoys a full meal from its host.

“So, though it can support the virus, it’s not as good at spreading it,” Beckham said. “It does have a wider range in the U.S. and moves farther up the coast, so if that mosquito population became infected to a large degree, then we could see a problem.”

How the mosquito behaves is not the only quality that’s important in the spread of disease. The behavior of the virus also plays a part.

In some cases, viruses have been known to change a mosquito’s behavior, such as turning it into a “sipper” when that wasn’t its eating habit before.

The CDC has set specific travel alerts and recommendations based on the behavior of the virus and its hosts. Visit cdc.gov/zika for updated information.

Zika virus was first discovered in 1947 and named after the Zika Forest in Uganda. But it wasn’t until 1953 that the first human infection was identified. Since, small outbreaks have occurred in tropical Africa, Southeast Asia and the Pacific Islands, according to the CDC. Its symptoms had always been mild and not associated with any long-term effects — until recently.

In 2015, the first confirmed Zika virus infection was announced in Brazil, after which Brazil announced an alarming increase in the number of babies born with birth defects from mothers who had been infected with Zika virus during pregnancy.

Brazil also reported an increased number of people with Guillain-Barre syndrome, an uncommon sickness of the nervous system in which a person’s own immune system damages the nerve cells, causing muscle weakness and possible paralysis, according to the CDC. As of June 1, those reporting to have contracted Guillain-Barre syndrome have not suffered any serious consequences, Beckham added.

“Something happened to Zika — a change in its genetics at some time,” Beckham said. “We knew about Zika (virus) before (the Brazil outbreak), but it wasn’t causing these concerns. Something changed in the early part of this century that made it more capable of spreading, and we are still trying to understand what happened to the virus for it to start producing these bad outcomes.”

The first step in studying such a complex issue is replicating it in the lab, Beckham said. Currently, he is working to get the virus into a pregnant host — a mouse — and study how the virus spreads to the fetus and other organs.

“You have to start with a good model,” he said. “Currently, we are developing a model of Zika virus infection in pregnant mice so that we can then study and hopefully understand how the immune system recognizes the virus and responds. If we can understand that, we can start developing vaccines.”

These studies could then further explain how the Zika virus affects pregnant women, specifically the fetus, at different stages of pregnancy; how it affects reproduction; the long-term infection risks; and the effect a second infection of Zika virus may have.

“To prevent mosquito bites, people should wear long-sleeved shirts and pants while outside when feasible, apply insect repellent when going outdoors, and use window screens or air-condition while indoors,” he added.