Narrator: This is Science Today. More and more drugs are being designed on computer. In rational drug design, as it's called, scientists design a drug molecule that binds to a specific site on a virus or bacteria, like a key in a lock, blocking the action of the disease. Biochemist Andrew McAmmon of the University of California, San Diego says the design process doesn't stop there.

McAmmon: One of the great virtues of rational drug design is that it may create a relatively effective way of responding to the appearance of resistant emergent strains of viruses or bacteria.

Narrator: When a virus or bacteria becomes resistant, what's happened is that the target area has changed size, so the drug molecule no longer fits. But rational drug designers can go to their computers and design a new drug that fits the mutated disease.

McAmmon: I think the whole nature of the pharmaceutical business in the 21st century is going rely very heavily on computers to track these moving targets and to re-engineer drugs to respond to the emergence of resistant strains.

Narrator: For Science Today, I'm Steve Tokar.