During the past several years, though, the field has shown a resurgence. “We’re about to enter a renaissance for vaccines that could easily rival the successes of the 1950s and 1960s,” says Gary J. Nabel, director of the Vaccine Research Center at the National Institutes of Health. Merck’s Gardasil, for example, is only one of the marvels generating substantial profits. The drug, which protects against certain strains of human papillomavirus that cause cervical cancer, produced $1.5 billion in sales last year. And more vaccines are on the way. In 2005 there were 68 in Phase I FDA trials, and 21 had progressed to Phase III. Eventually, there might even be vaccines to prevent such major killers as heart disease, which some researchers now believe may have infectious roots. And while vaccines still represent less than 3% of overall drug company revenue, analysts expect the vaccine industry to grow by more than 10% a year through 2012, compared with 7% for pharmaceuticals overall.

Danzon attributes this comeback to renewed demand, scientific advances and a suddenly favorable business climate. The major pharmaceutical companies, she notes, have at least temporarily run out of blockbuster drugs to pursue. “They’ve picked all the low-hanging fruit,” she says. “Now they’re beginning to look harder at drugs with smaller potential revenue.” That has brought vaccines back into the picture.

One driving force behind the renewed interest in vaccines can be summed up by a single word: fear. Because vaccines prevent rather than treat diseases, they tend paradoxically to mute the alarms that might arouse public demand for innovation. When children aren’t being crippled by polio or killed by measles, people forget about lurking viral threats. “Until recently, vaccines have been undervalued,” says Richard M. Haupt, executive medical director for Merck. “We spend less than 5% of our health care dollars on preventive measures, including vaccines. The vast majority of spending is on treatment.”

Threats of terrorism have challenged that sense of complacency. Because contagions could be delivered in tiny, easily concealed containers, detection may be impossible, and attempts to treat victims of an attack might prove too little, too late. “The only rational way to combat agents used by bio-terrorists would be with vaccines,” says Michael Starnbach, associate professor of microbiology and molecular genetics at Harvard Medical School.

In 2004 President Bush signed into law Project BioShield, which authorizes spending $5.6 billion over 10 years for safety measures, including stockpiling and improving existing vaccines and developing new ones against anthrax, smallpox and other agents. And Nabel says that since the anthrax attacks in 2001 and outbreaks of West Nile virus, severe acute respiratory syndrome (SARS) and avian flu, it has hit home that more must be done to head off potentially catastrophic scourges. “As the world becomes more crowded, we’re facing threats from biological agents we never imagined,” he explains. “And the natural ones may be worse than the deliberate biodefense threats.”

Even conventional flu vaccines are getting a shot in the arm. After well-publicized shortages, the federal government has tried to quicken the flow, awarding $1 billion in the spring of 2006 to help drug companies modernize production. -Novartis is using a $220 million contract to build a flu vaccine plant in North Carolina, while GlaxoSmithKline is refurbishing a Pennsylvania plant under a $274 million federal contract.

As demand for vaccines has increased, so has researchers’ understanding of how the human body fights disease—and of ways to turn that process to further advantage. Unlike bacteria, which are complete cells that live both inside and outside humans and animals, viruses are tiny, incomplete pieces of genetic material that can neither survive nor reproduce away from a host. Essentially a piece of DNA coated in protein, a virus enters the body through an orifice or cut, attaches itself to a cell and injects its genetic material.