The symptoms were clear but the cause was a mystery: lethargy, headache, itchy eyes, fatigue and occasional breathing problems that suddenly appeared among office workers every morning and miraculously disappeared on the way home every evening.

Clearly, some people thought, the problem was a case of office blues suffered by disgruntled employes who really didn't want to do their jobs. But in fact, the workers were grappling with a bout of "sick building" disorder caused by mold growing in the air conditioning ducts.

Just like people, buildings also get "sick," conclude reports presented last week at the American Society for Microbiology annual meeting at the Washington Convention Center. Given the right conditions, experts said, any kind of structure -- from hospitals to single-family homes to skyscrapers -- can harbor disease.

And these "sick buildings" often spread their diseases to unsuspecting occupants.

"We're only beginning to scratch the surface of this problem," said Harvard Medical School epidemiologist Edith Kundsin, who chaired the symposium on architecture and health.

"No one really has a good idea of how many buildings have these problems," said Philip Morey, a senior industrial hygienist with Honeywell's new Indoor Air Quality Diagnostics division. A small percentage of buildings, perhaps just 2 or 3 percent, contain asbestos and radon, Morey estimates.

The majority of structures have the more minor, but often irritating, problems of maintaining comfortable temperatures and good air quality in the face of changing seasons and cigarette smoke.

About "10 percent of sick buildings or less have microbial infections," said Morey, who is a former microbiology researcher with the National Institute of Occupational Safety and Health (NIOSH). These viruses, bacteria and molds lurk in unsuspected places and can account for allergies (including sinus trouble), flu-like outbreaks, respiratory problems and other symptoms.

In one office building, located in a southern city, three suspicious outbreaks of a flu-like disease afflicted 40 to 50 percent of its occupants, Morey said. Each outbreak lasted only a few days. After the third bout, NIOSH investigators traced the problem to a 6,000-gallon water tank. Every weekend the water in the tank was stagnant, creating an ideal breeding ground for bacteria.

"Microorganisms can grow on concrete block, ceiling tiles and things that you would not consider very nutritious," Morey said. "They're capable of using dirt, debris and cellulose as food."

Outbreaks of illness spread via "sick buildings" have been traced to ceiling tiles, carpeting, leaky dishwashers and air conditioning ducts, Morey said. Even in buildings that are air conditioned during the summer, rising humidity can help mold and bacteria thrive in carpeting. Steam-cleaning rugs may not help, since that does not remove dead organisms, which can also provoke allergies.

"The problem is that people can be allergic to the dead material in the carpet." Morey said. And if rug cleaners are unsuccessful at getting all the live organisms out of the carpet, he said, "there is a danger that these organisms can reseed or regerminate and grow again."

Then there is humidifier fever -- or hypersensitivity pneumonitis -- a disease caused by a variety of microorganisms that flourish in humidifiers or other moist areas with little ventilation. The illness can cause fever, chills, chest tightness and wheezing. Chronic exposure to humidifier fever and other "sick building" diseases can produce a far more serious disease -- a thickening of the lung lining known as fibrosis.

The "sick building" syndrome is particularly troublesome in hospitals. One medical institution experiencing a high rate of infections "traced the problem to an air duct," said Dr. Carl Walter, clinical professor of surgery emeritus at Harvard Medical School and an investigator of "sick" hospitals. "The air intake duct for the hospital was placed so that pigeons could roost in it. We found six to eight dead pigeons in it that were causing the problem."

"Most of the 'sick' hospitals that I've investigated have been the result of bad architectural design, stupid construction and faulty maintenance," Walter said.

Among the most widely publicized outbreak of building-related illnesses is Legionnaire's disease, the deadly bacterial infection first discovered in a Philadelphia hotel in 1976. Twenty-nine people died in that outbreak, and 153 others were infected, apparently from stagnant water sitting in a cooling tower atop the hotel.

At any one time, about 20 percent of a building's occupants will feel either too hot or too cold, Morey said. But when complaints expand to other physical ailments and affect 60 to 70 percent of occupants, he said, "there is no question that you've got a 'sick building.' "

Among the symptoms associated with "sick buildings" are sinusitis, eye irriation including trouble wearing contact lenses, runny noses, headaches, nausea and rashes. "If you have several of those symptoms, and the pattern repeats itself and you feel better when you leave the office, you should investigate that," Morey said. More News From The Microbiology Conference

Among other news reported at the American Association of Microbiology's annual meeting:

*A "promising" new vaccine to fight one of the main causes of spinal meningitis in the United States is being developed. The Food and Drug Administration announced that testing has begun on this new vaccine to protect against a bacteria called group B meningococcus. If it works, the vaccine, which has been developed by the FDA and Baylor College of Medicine in Houston, will be first to protect against a strain of the disease that primarily affects very young children.

*Rising numbers of gastrointestinal infections are caused by viruses that can slip through water processing systems, contaminate shellfish and easily spread from person to person, reported researchers from the Centers for Disease Control, the National Institute of Allergy and Infectious Diseases and the Children's Hospital National Medical Center. Communities that use rivers as a source of drinking water "must be able to contend with solids-associated virus in their water treatment plants," the researchers said. Public water systems test regularly for bacteria in the water, but are not required to test for viruses.

*Two new vaccines designed to prevent malaria are being tested in human clinical trials, with promising preliminary results, researchers reported. One vaccine has been developed jointly by Walter Reed Army Institute of Research, the National Institutes of Health, the Naval Medical Research Institute and the Smith, Kline and French Laboratories. The other vaccine is a product of research by New York University and Hoffman-LaRoche Laboratories.