IT IS widely assumed that the evolutionary struggle is all but over and that the human race has won. The lions and tigers and bears that once chased us up trees or kept us cowering in caves now are dependent on our good will for their continued existence. We act as though we've pretty much got control of the planet except for the weather and tides, and we're working on them.

Don't you believe it. As a really, really scary new Smithsonian exhibition demonstrates, we are locked in a deadly and perhaps losing struggle with an unseen enemy that constantly changes its shape and tactics. The enemy is the class of tiny organisms called microbes, which saturate the air, land and sea -- and our bodies -- in such numbers that if they all came out of hiding they would bury Earth's landmass five feet deep. Bacteria, fungi, protozoa and viruses were here first, by billions of years, and they're odds-on favorites to outlast us.

What's more, many of the microbes that aren't sickening and killing us are keeping us alive. They enrich the soil, cleanse the waters, and generally create the conditions necessary for the "higher" orders of plants and animals to sustain life. The poet's loaf of bread and jug of wine are both made possible by yeasty beasties, and we can take nourishment from them only with the aid of bacteria that swarm in our guts. Whether we can continue to live with microbes remains to be seen, but there's no question that we cannot live without them.

The microbe exhibit, which is appropriately installed in the Smithsonian's underground International Gallery, is a thunderous wake-up call and a crafty recruiting campaign. It has all the bells and whistles: interactive and 3-D video, virtual reality war games, a skeleton-filled catacomb, holograms of magnified microbes and the scariest costume since Darth Vader's. While the show's designed to make all of us take health and sanitation more seriously (at the very least, to wash our hands more often), the main purpose is to draw young people into the ranks of white-coated doctors, nurses, researchers and technicians who stand between us and disaster. Microbiology is a guaranteed growth profession.

The enemy is formidable indeed. Among the bad bugs we're battling are bubonic plague, cholera, dengue fever, Ebola, giardia, hepatitis, herpes, HIV, influenza, malaria, polio, salmonella, streptococcus, trachoma and tuberculosis, diseases whose causes, symptoms and consequences are recounted in graphic detail. There are many other diseases, of course, ranging from troublesome to deadly, and an endless roster of players to be named later: More than 30 newly recognized infectious diseases and syndromes have emerged in the past 20 years. Almost every time we develop a new way to lock an invading organism out, it develops a new key. Miracle drugs work miracles until the microbe mutates, and there now are a number of microbes that are immune to every shot in our locker.

Waiting in the wings is smallpox, an ancient and possibly a future scourge of mankind. The disease was declared extinct -- humanity's first and only total victory in the germ wars -- in 1977, when the last human carrier was isolated until the infection had run its course, depriving the virus of its last host. But instead of destroying the remaining laboratory stocks of the virus, which had been used to make vaccines, the United States and the Soviet Union kept samples. People are no longer vaccinated; if the virus is accidentally or purposely loosed, it can be expected to sweep the planet as never before.

For visitors of a certain age the most affecting item in the exhibition is the child-size iron lung. Many of us watched siblings or friends wither and die in these sinister metal-and-glass canisters, and those of us who learned to walk again, and went on to have children of our own without fear of polio, revere the memories of Drs. Salk and Sabin.

The show is wonderfully well done and is being enthusiastically received by the kids who flit through it, being painlessly and almost subliminally indoctrinated in the evils of the Invisible Empire. "Ewww, gross!" is the most commonly heard comment, and proves the kids are paying attention. The real hook-setter is a little shop of horrors at the exhibit exit. It sells books with titles such as "Gross Ology," which has simulated vomit on the cover and promises to tell all about boogers and spit and such. Another is the "Almanac of the Gross, Disgusting and Totally Repulsive," featuring feces, pus and snot. There are also slime-making kits and cootie-covered tote bags full of Gummi Worms; everything, in short, to delight the heart and capture the mind of a youngster.

MICROBES: Invisible Invaders . . . Amazing Allies -- Through Sept. 6 in the International Gallery at the S. Dillon Ripley Center, 1100 Jefferson Dr. SW (Metro: Smithsonian). 202/357-2700 (TDD: 202/357-1729). Open 10 to 7:30. Wheelchair accessible. Web site: www.si.edu.