A study released Wednesday supports the argument that infections during pregnancy may cause some cases of autism.
Women who had active infections with genital herpes early in pregnancy were twice as likely to have a child with autism than women who did not, a team of researchers found.
The findings, published in the journal mSphere, add to evidence that inflammation during pregnancy may affect the brain of a developing fetus.
"We believe the mother's immune response to HSV-2 (herpes simplex type 2) could be disrupting fetal central nervous system development, raising risk for autism," said Milada Mahic, a researcher at Columbia University who led the research team.
Viruses can infect a developing baby's brain — Zika is the most notorious now, but cytomegalovirus and rubella also cause severe birth defects, including brain damage.
But that's not what's going on with autism, said Dr. Ian Lipkin, an epidemiologist and infectious disease expert at Columbia who oversaw the research.
"Most people, when they think about viral infections, think in terms of viruses going into tissue, killing cells, damaging cells," Lipkin told NBC News.
But Lipkin believes that, in fact, it's the mother's immune response — the inflammation — that's causing the damage. It may be that inflammatory chemicals such as cytokines are crossing the placenta and affecting the developing brain of the fetus, he said.
For the study, Lipkin's team started looking at women who were pregnant in Norway in the 1990s. They found 442 mothers of children with autism and compared them to 464 women who had babies without autism in the same months.
The women were tested for immune responses to four viruses known to cause birth defects: cytomegalovirus, rubella, herpes simplex 1 (HSV-1, which causes cold sores) and HSV-2, also known as genital herpes. The women were also tested for the toxoplasmosis parasite.
The women with high levels of antibodies to HSV-2 midway through their pregnancies were twice as likely to have a baby later diagnosed with autism. None of the other viruses seemed to affect autism risk.
"The elevated antibody levels to HSV-2 may indicate either recent primary infection or reactivation of latent infection," the researchers wrote.
Herpes infections are incurable and can flare up throughout a person's life. They are also very common, so having been infected with herpes does not mean a woman is going to have children with autism.
However, autism is becoming diagnosed more commonly.
Surveys by the Centers for Disease Control and Prevention find that as many as than 2 percent of U.S. kids have been diagnosed with autism — anywhere between 1 in 68 and 1 in 45 children.
The autism spectrum refers to a broad range of symptoms, from the relatively mild social awkwardness of Asperger's syndrome to profound mental retardation, debilitating repetitive behaviors and an inability to communicate.
There's no cure and no good treatment.
A 2013 study found that women who had flu while they were pregnant were twice as likely to have a child later diagnosed with autism. Those who had a fever lasting a week or longer — perhaps caused by flu or maybe by something else — were three times as likely to have an autistic child.
"A wide range of infectious agents has been linked to autism spectrum disorder (ASD), suggesting the possibility that general immune activation in susceptible subjects, rather than a specific pathogen per se, is associated with risk of ASD," the Columbia team wrote.
"We speculate that ASD risk associated with high levels of antibodies to HSV-2 is not specific to HSV-2 but instead reflects the impact of immune activation and inflammation on a vulnerable developing nervous system."
The team found the risk if a woman had signs of herpes infection only during the first trimester of pregnancy, when the brain of the fetus is developing quickly. It also did not have enough girls with autism in the study to be able to tell whether the risk is also higher for girls. Boys are far more likely to be diagnosed with autism.
"We are now looking at other triggers. We think that a wide range of different types of infections can cause this," Lipkin said.
"I think that this makes a very strong case for continuing to try to develop herpes simplex vaccines."