On an airline flight not long ago, Julie M. Scofield, executive director of the National Alliance of State and Territorial AIDS Directors, fell into the kind of a conversation many people have with the person in the next seat over. She asked what he did for a living; he asked what she did. There was a silence. Then he said, "Tell me: Is it really true you can't get HIV from saliva?"

Some 20 years after scientists identified HIV as the virus that causes AIDS, most people seem to know the basics of how it passes from person to person. But confusion over the details of transmission is still common.

"We really find it is true that people are unsure about transmission," says Susan L. Fulmer, immediate past chairwoman of the HIV/AIDS section of the American Public Health Association and a public health official in South Carolina. "We still get calls on our state AIDS hotline: 'I sat on a toilet seat and when I got up I saw there was some blood there,' or 'Somebody spat at me.' But quite honestly," she says, "what people most often have to be concerned about is having unprotected sex or sharing drug paraphernalia."

Scofield agrees. "The most important thing for people to know is that consistent and correct use of condoms is a very effective way to prevent transmission of HIV. The second thing is you've got to know your (sexual) partners."

According to the Centers for Disease Control and Prevention, the most common ways HIV passes from an infected person to an uninfected person are during unprotected vaginal, oral or anal sex, through shared use of injection equipment, and from an HIV-infected mother to her baby before birth, during birth or after birth while breastfeeding.

A 2004 survey conducted by the Henry J. Kaiser Family Foundation found the great majority of people questioned understood most of this: 99 percent knew that HIV can be transmitted by having unprotected intercourse and by sharing injection needles; 91 percent understood the virus can be transmitted by having unprotected oral sex. But almost four in 10 people who took the survey said it was either possible or they were unsure whether HIV can be transmitted by kissing. A quarter thought the virus might pass between people who use the same drinking glass. And almost one in five thought touching a contaminated toilet seat might lead to infection.

"You really have to sit down and look at these situations," says Fulmer. "Often, the things people seem to be most concerned about are the things that aren't as likely to cause trouble." With that in mind, here are some answers to questions about how HIV is—and is not—transmitted.

Can you get HIV from saliva? How about tears or sweat?

According to the CDC, researchers have found HIV in the saliva and tears of some infected people. But the quantity of the virus in these fluids is very low, and the CDC has found no cases in which HIV has been transmitted to an uninfected person in saliva or tears. HIV has not been found in sweat, and sweat has never been found to transmit HIV.

Can you get HIV from deep, or open-mouth, kissing?

"If an infected person has bleeding gums, then yes, there's the possibility of transmission," says Fulmer. But, she adds, skin is a good barrier. To pass on the virus, it's likely an infected person would have to have blood in his or her mouth and that blood would have to pass into the body of a partner through cuts or sores in the partner's mouth.

The CDC reports one case that suggests a woman became infected after being exposed to her partner's blood while kissing with open mouths. Because deep kissing can possibly lead to cuts in the mouth and exposure to blood, the agency recommends against open-mouth kissing with a person who has HIV.

Can you get HIV from a mosquito bite?

No. HIV does not survive in mosquitoes or other insects. Insects don't become infected and can't transmit the virus to humans.

Can you get HIV after being stuck by a contaminated needle?

Being stuck by a contaminated needle can transfer HIV. But the risk of transmission is very low and the CDC does not know of any cases in which HIV has been passed on by a needle-stick injury outside of a healthcare setting. Even so, if you are stuck by a needle, the CDC recommends that you see your doctor or go to an emergency room as soon as possible.

Can you get HIV while getting a tattoo or body piercing?

Since HIV is in the blood of infected people, it's technically possible that instruments used for tattooing or body piercing could be contaminated and might pass on the virus if they aren't sterilized or disinfected between uses. If you are thinking about getting a tattoo or a body piercing, ask the staff at the shop what they do to prevent the spread of infections.

Can you get HIV by drinking from a glass or using a toilet after an infected person?

No. The CDC calls using the same glass, shaking hands, getting a hug and similar behavior "casual contact," and casual contact does not transmit the virus. HIV does not survive long outside of the body, and it isn't transmitted in the air or in food.