THERE'S A LOT OF SICK ART in this world.

Paintings, sculpture and works on paper suffer from hidden diseases that sleuths uncover in the laboratories of major art galleries. The etiology of some of these illnesses is discussed at length in "Conservation of Modern Art," opening Saturday at the Hirshhorn Museum.

Reading the diagnoses and treatments in these 17 absorbing case studies is enough to make you wince and swear you're in the Armed Forces Medical Museum. Causes of damage range from bungled attempts at restoration and the effects of light and pollution to simple naivet,e or indifference on the part of the artist using modern materials.

For example, they knew something was wrong with Antoine Pevsner's "Head of a Woman" when the plastic sculpture was observed to be dripping into its case. "Head" was made in 1923 of interlocking pieces of cellulose nitrate (the same villainous material as in some rapidly deteriorating old films). The foot-long "Head" had been sealed in a clear display case for its protection. But the closed case actually speeded up the sculpture's deterioration, because destructive acid was able to accumulate there.

Hirshhorn diagnosticians prescribed changing the case to one with holes for ventilation, and keeping the temperature of the surrounding air at a benign 68 degrees. It's not a cure -- "Head" is still turning brown from exposure to light and crumbling into nothingness -- but its lifetime has been extended.

The remedy, as in the case of Childe Hassam's "The Union Jack, New York, April Morn," may be little more than removing a yellowing varnish to reveal the artist's true impressionist colors underneath. Or it could be applying bandaids, as technicians did with the extruding paint on Randall Davey's "Young Girl in Blue Top."

Humidity had caused the painting's linen support to shrink, so the paint popped out in little tents. In the rescue, the paint had to be held on to the canvas with makeshift bandages -- strips of tissue and paste; then a wax resin adhesive was combined with the paint, like a blood transfusion.

The show documents the Hirshhorn's conservation efforts by displaying photos next to the refurbished works of art. In a "before" photo of an Arthur B. Davies' landscape, "Fiesole," the clouds on an otherwise fine day are disturbingly black, not what Davies intended. But he didn't know from Chem 101 that the white lead pigment he used changes to black lead sulfide when exposed to (what else?) sulfur dioxide. In the lab, the black sulfide layer was oxidized to a white sulfate, and the clouds are cumulus again.

If you're a collector, you'll also learn in a slide show here how to preserve your outdoor sculpture, which has to endure the indignities of wind, rain, sun and bird droppings.

If you're an artist, you'll be reminded that commercial cellophane tape turns yellow and falls from collages, and that there are few secrets from scholars with infrared cameras that can see just under a painting's surface.

CONSERVATION OF MODERN ART -- Opening Saturday at the Hirshhorn Museum and Sculpture Garden, through March 31.