It takes only a sneeze, a cough or a handshake to spread cold or flu germs from one person to another. But emotions can be transmitted even more easily, faster than the blink of an eye.

Research has found that emotions -- both upbeat ones like enthusiasm and joy, and negative ones like sadness, fear and anger -- are easily passed from person to person, often without either party's realizing it. Emotional contagion occurs in a matter of milliseconds, says Elaine Hatfield, a professor of psychology at the University of Hawaii and co-author of "Emotional Contagion" (Cambridge University Press, 1994). If you're the receiver, you may not know what exactly happened, just that you feel differently after the encounter than you did before.

It turns out this phenomenon depends on a basic, even primal, instinct: During conversation, humans unconsciously tend to mimic and synchronize the other person's facial expressions, posture, body language and speech rhythms, explains John T. Cacioppo, professor of psychology and director of the Center for Cognitive and Social Neuroscience at the University of Chicago.

When it comes to this monkey-see, monkey-do dynamic, "the more expressive and sincere someone is, the more likely you are to see that expression and mimic it," Cacioppo says. "The muscle fibers [in your face and body] can be activated unbeknownst to you, at much lower levels than if you were to express those movements yourself initially."

In a study at Uppsala University in Sweden, researchers exposed people to pictures of happy or angry faces for 30 milliseconds, immediately followed by neutral faces. Even though the participants didn't realize they'd just looked at a happy or angry face, they responded with distinct facial muscle reactions of their own that corresponded to the emotion they'd just seen.

Those incremental muscle movements then trigger the actual feeling by causing the same neurons to fire in the brain as if you were experiencing the emotion naturally, according to Hatfield. In other words, the mood feedback loop can travel in both directions: Normally, when you feel happy, your brain might send a signal to your mouth to smile. With the mood-contagion effect, the facial muscles involved in smiling might begin to twitch when you're with a cheerful friend and those tiny muscle movements then send a signal to your brain, telling it to feel happy.

But there may be another mode of transmission: In the course of conversation, people have a tendency to match the emotional tone of their word choices -- particularly when it comes to using negatively charged words such as "hate," "worthless," "anger" and "sad" -- with the tone being used by whomever they're talking to, according to research presented at the Midwestern Psychological Association's annual meeting earlier this month.

"Communication requires the matching of specific words and contents so [people] can understand each other," explains study co-author Frank Bernieri, an associate professor of psychology at Oregon State University in Corvallis. "So it's not hard to imagine the language driving some part of this contagion process."

Whether it happens at home, work, school or other settings where you have close interactions with other people, this communicative dance is highly adaptive and functional, experts say, because it allows you to know what other people are feeling or thinking. "It's the very first idea of mind reading," Hatfield says. "For the vast majority of people, you want to know what other people are thinking, to be in sync with them, to have sympathy and empathy."

Not only can this phenomenon help people connect on a very basic emotional level, but it has practical applications, too, including survival value. "If there was some emergency like [you are part of a group of people] about to be hit by a car, I would want to be able to catch their emotions because they're signaling things that are critical to my survival," Cacioppo says.

Being able to read and experience other people's emotions also has benefits in the face of conflict (such as a marital spat) or competition (such as a sporting event).

And it may be useful in work settings -- if the moods swing the right way. In a recent study, researchers at the University of Michigan found that when business leaders were in a good mood, members of their work group experienced more positive and fewer negative moods.

They also found that groups with leaders in an upbeat mood were more coordinated and expended less effort on tasks than groups with more downbeat leaders.

The degree to which people become emotionally in sync with each other depends partly on the level of intimacy and emotional investment in their relationship. Not surprisingly, people living under the same roof are especially likely to catch each other's moods.

In a study at Northwestern University, researchers periodically induced and assessed emotional responses in both dating partners and college roommates: Over the course of a year, they found that people in both types of relationships became more similar in their emotional responses. Among those who were dating, the partners who experienced emotional convergence had relationships that "were more cohesive and less likely to dissolve," the authors concluded.

Meanwhile, research at the Arlene R. Gordon Research Institute in New York found that among elderly couples, when one spouse is depressed, the other is likely to experience similar symptoms. And a study at the University of Texas Medical Branch in Galveston found that depression was highly contagious among college roommates, with those who sought a lot of reassurance and support from their depressed roommates being particularly vulnerable to catching the feeling.

But even mere acquaintances can catch each other's moods, depending on their individual susceptibility. Peter Klaus, a public relations professional and an actor in the District, has experienced this with a neighbor in his apartment building. "Whenever I bump into this woman, while taking out the trash or waiting at the elevator, she always has a smile and an openness and pleasantness to her," he explains. "And it makes me want to respond the same way. After talking to her even for a minute, I feel happier and more outgoing."

While some people are more prone to infecting others with their moods, others are more likely to become engulfed by people's emotions. People who are more expressive -- meaning, they wear their hearts on their faces and their sleeves -- may be more likely to spread their emotions because they telegraph their feelings more powerfully. On the other hand, it appears that people with high autonomic reactivity -- they respond strongly internally to emotional events (their hearts may race when they're nervous though they seem calm on the outside) -- may be more susceptible to catching other people's moods, Cacioppo says.

With any luck, people catch the positive emotions -- a colleague's enthusiasm for a project at work, a friend's excitement over an athletic event -- and miss the negative ones. Some psychologists suspect, however, that negative emotions may be more infectious. "If someone is sharing negative emotions in a self-disclosing, personal realm, you have to be empathic and acknowledge it," Bernieri says, which makes the emotion more likely to spread.

Marta Wiseman, a mother of two in Oakton, has experienced this firsthand. "I try to start my day feeling hopeful and positive," she says, "but some mornings my 15-year-old mopes around, complaining about having to get up so early, and his disposition brings me down."

While the idea of making yourself impervious to other people's emotions may be appealing, putting up an emotional barrier is not the answer.

"There's a cost to it -- you lose empathy," Cacioppo says. After all, shutting out other people's dark moods precludes you from catching their good cheer, too. ·

Stacey Colino regularly writes about psychological issues for the Health section.