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They’re the quiet ones in class. The ones that stare at the ground and avoid eye contact. They shrink away from large crowds, preferring the company of a few close friends.

In the world of introverted personality types, silence reigns.

Scientists have documented the full consequences of this silence – introverted people are more likely to develop infectious diseases, including AIDS.

Physicians in ancient Greece took note that those with a “melancholic temperament” were more susceptible to viral infections, including the flu.

Additionally, introverted people recover more slowly from these illnesses than people with extroverted personality types, or those that are more lively and outgoing.

No one knew why.

But now, UCLA scientists think they have made an important discovery that may shed some light on this centuries-old puzzle.

“We were interested in trying to understand the biological mechanism that mediates psychological influences on infectious vulnerability,” said Steve Cole, principal investigator and assistant professor of hematology-oncology at the UCLA David Geffen School of Medicine.

Cole’s team of scientists discovered that introverts had higher levels of autonomic nervous system activity.

The ANS is responsible for regulating automatic processes such as breathing, heart rate and blood pressure.

It was known that chemicals released from this system can accelerate HIV replication in the test tube, Cole said.

“We wanted to find out whether a similar relationship held in a clinical study,” Cole said.

The team studied a group of 54 men who were in the middle stages of HIV, focusing on the effects of stress on viral replication.

Stress stimuli included beeping noises at unexpected times and physical exercises such as deep breathing and standing from a seated position.

The men studied were asked to solve math problems under time constraints.

They found that men clinically diagnosed as being introverted progressed toward AIDS faster than extroverted men.

Additionally, the introverted men did not respond as well to AIDS treatment drugs.

“Shy, introverted people showed eight-folds higher levels of virus in their blood,” Cole said.

The dramatically different levels of the virus were due to the higher levels of ANS activity in introverted individuals, said Cole.

Their findings show that the ways in which people respond to different stress stimuli account for the differences in the ANS.

The factors that determine whether an individual will be introverted or extroverted seem to be a combination of nature and nurture, said Marian Sigman, a UCLA professor of clinical psychology.

“We think that children’s environment affects development as well – not just that they’re born (shy),” she said.

Should introverted individuals, then, be medically treated to become more outgoing?

Not necessarily, Cole said.

Researchers are unsure whether a change in personality will cause a different response to stress stimuli and also believe that being introverted yields many positive things.

“(Introverts) are disproportionally responsible for our greatest cultural achievements,” Cole said.

“Anything that requires depth and intensity, introverts are better than extroverts,” he added.

His team is now in the process of testing drugs that can block an introverted person’s ANS response to stress.