A vacationing couple fears the popcorn they bought at a gas station is contaminated. A woman panics when an employee sneezes while scooping her ice cream. A man wonders if he could be infected by shaking hands. Some worry about using public restrooms or hot tubs or being bitten by mosquitoes.

More than 3,000 times every day people with concerns like these call the toll-free National AIDS Hotline. The world's largest health information telephone service, the hotline has fielded nearly 5 million calls since 1987. Hotline staff say that many of the callers are panicked and seek reassurance and information about whether the disease that has killed more than 100,000 Americans since 1981 can be casually transmitted.

Recent studies show that a surprisingly large segment of the American public continues to reject expert assurances that AIDS is not spread by contact with saliva, toilet seats, doorknobs or insects. There are also signs of increasing confusion about the best ways to prevent the disease.

"Even with relatively high information levels about how AIDS is transmitted, misperceptions about casual transmission persist," said Ann M. Hardy, a researcher with the federal government's National Center for Health Statistics (NCHS). In the past three years, the NCHS has conducted an ongoing national survey of a total of 100,000 American adults on the extent of their knowledge about AIDS. Results are released every three months.

While about two thirds of adults score high on overall AIDS knowledge, one third still answer questions about casual transmission incorrectly, Hardy said.

"As much as 40 percent of the population has a substantial level of miscomprehension about the spread of AIDS," agreed James A. Wells, who conducted an international survey of AIDS knowledge and behavior for Project HOPE, a Washington-based health education and research organization. The elderly, minorities, people with lower levels of education and those for whom English is a second language lag farthest behind, according to Wells.

Below are some recent findings from both surveys:

General knowledge. There is virtually universal knowledge of the three main modes of transmission. A recent NCHS report, based on data through mid-1990, found that 87 percent of adults definitely know that AIDS is transmitted through sexual contact, 95 percent cited needle-sharing among intravenous drug users while 85 percent knew that the virus can be passed from an infected pregnant woman to her baby.

Newer areas of AIDS knowledge are also better understood. Forty-two percent in the 1990 NCHS survey knew that "AIDS can damage the brain," an increase over 26 percent from mid-1989. And 71 percent of adults understood that the term "HIV" refers to the virus that causes AIDS.

But as the amount of AIDS information proliferates and becomes more complicated, some Americans may feel they know less. Adults who said in mid-1990 that they knew a lot about AIDS declined to 19 percent from 24 percent in late 1989, while those claiming to know very little increased to 11 percent from 7 percent in the 1989 NCHS survey.

Prevention. There was a decrease in the percentage who correctly knew that a vaccine against AIDS was not available and "increasing uncertainty" about preventing AIDS through the use of condoms, said NCHS researchers. Those who think condoms are "very effective" in preventing transmission dropped from 33 percent in late 1989 to 27 percent in the second quarter of 1990. The number who did not know whether they were effective rose.

This may reflect mixed messages from AIDS educators: While most call condoms the best preventive method -- short of abstinence -- they have increasingly emphasized that the devices are not 100 percent effective.

Casual transmission. Both surveys found similar -- and significant -- knowledge gaps about casual transmission. In the NCHS 1990 findings, the percentage of adults who correctly said that it was "very unlikely" or "not possible" to get AIDS or AIDS virus infection from eating in a restaurant with an HIV-infected cook was only 55 percent; from sharing utensils with someone who is infected, 47 percent; from being coughed or sneezed on by an infected person, 47 percent; from mosquitoes or other insects, 43 percent; from using public toilets, 61 percent.

The public was more confident that AIDS could not be transmitted by working or attending school with an infected person. In each case, about three fourths of those surveyed said such transmission was very unlikely or impossible.

HIV testing. The NCHS 1990 AIDS surveys found that 79 percent of all adults had heard of the blood test for HIV and an extraordinary number -- a figure estimated to represent about one fourth of all adults, about 45 million Americans -- have been tested for a variety of reasons.

The largest group -- 15 percent of adults -- have undergone HIV testing as a result of donating blood since 1985. Since then, blood donations have been routinely screened and discarded if there is suspicion that a donor might be infected with HIV. Two percent of those who had donated blood said they did so at least in part to be tested for the virus, a practice blood banks strongly discourage.

Ten percent said they had been tested in a doctor's office, clinic or other site. Reasons included hospitalization or surgery, military induction, application for life or health insurance, or for personal knowledge of their HIV status.

HIV testing was more common among those under 29 and among blacks and Hispanics. Three fourths of those tested received their test results, but less than one third were counseled about AIDS prevention.

There is still widespread skepticism about the safety of the blood supply; only half of those surveyed believe it is safe. The Project HOPE survey also found that 40 percent of adults mistakenly believe that HIV is transmitted by donating blood.

Personal experience. NCHS found that most Americans -- 80 percent -- say they believed there was no chance they had been infected with HIV; 15 percent said they thought there was a slight chance. Fewer than 1 percent said they thought they had a high chance of being infected now or in the future.

Fifteen percent in 1990 said they know someone with AIDS or HIV infection, more than double the 6 percent in 1987. Two out of three say they discussed the disease with their children; 75 percent said their children had received AIDS instruction in school, more than twice as many as in 1987.

AIDS information. Ninety percent of adults had received information about AIDS in the month prior to the NCHS 1990 survey; television was the primary source (80 percent), followed by newspapers (57 percent), magazines (45 percent) and radio (33 percent). Project HOPE found that nearly 25 percent of sexually active adults say they reduced their number of sexual partners and/or used condoms; 6 percent said they were sexually abstinent.

A 1990 Project HOPE report, based on 1988 data, concluded that there was "clear evidence" that exposure to AIDS information can improve understanding and change behavior but that so far education campaigns have had only "limited success" in doing so.

"Unfortunately, the public is growing tired of hearing about AIDS," said Mary F. Cotton, director of HIV-AIDS Education for the American Red Cross. "The challenge now is many people believe they have all the facts they need, and clearly they don't."

AIDS Hotline Number

The federal government funds the National AIDS hotline, a 24-hour, toll-free, confidential service (1-800-342-AIDS).