They are so tiny that nearly 500 million of them could fit on the capital "t" in this sentence.

And they're alive.

They are known as bacteriophages, and Roger Hendrix has devoted his life to studying them.

Dr. Hendrix, who has been a biological sciences professor at the University of Pittsburgh for 36 years, is one of the world's leading experts on phages, which are viruses that infect bacteria and their close relatives, the archaea.

He delights in telling people that they are the most numerous creatures on Earth, and in coming up with creative statistics to demonstrate that.

There are an estimated 10 quadrillion quadrillion bacteriophages on the planet, he says. If each one were the size of a typical beetle, they would cover the surface of the globe to a depth of 30,000 miles. If actual-size phages that have tails were laid end to end, they would stretch for 200 million light years.

But there is a lot more to phages than the fact that there are scads of them, said Dr. Hendrix, who did his doctoral work at Harvard University under the tutelage of James Watson, co-discoverer of DNA.

Because of how they infect bacteria, phages play a big role in how adaptable bacteria are, and that in turn plays a big role in what happens to us.

When phages infect bacteria, they do one of two things, he said -- they either make copies of themselves and then burst the bacterium open so they can move on to new targets, or they quiet themselves down and incorporate their DNA into the bacteria's genome.

"Like any other organism," Dr. Hendrix said, "their imperative is to successfully reproduce. So sometimes that means they kill bacteria, and sometimes it means they take up with the bacteria and get replicated by the bacteria and pay their rent by providing some sort of selective benefit through their genes."

One of those benefits -- not to us, but to the bacteria they inhabit -- is to give the bacteria genes that make them more toxic, which means the bacteria can more easily invade human cells and get nutrients from them to reproduce.

In the case of such bacterial scourges as botulism, which can paralyze muscles; diphtheria, which causes heart and nerve damage; and cholera, which creates severe diarrhea, the toxic effects are all worsened by genes that originally came from phages and were incorporated into the bacteria, he said.

Phages don't always cause us harm, though, and in fact, some researchers feel they could become important tools in helping to diagnose and eradicate disease.

One of Dr. Hendrix's colleagues, Pitt professor Graham Hatfull, recently teamed up with researchers at the Albert Einstein College of Medicine at Yeshiva University in New York to use phages as an early detection method for deadly tuberculosis.

By attaching genes for a green-glowing fluorescent protein to the DNA of phages that infect the tuberculosis bacterium, the researchers have created a way to quickly detect TB infections in out-of-the-way clinics.

Other researchers are working on ways to engineer phages so they will kill bacteria that have become resistant to antibiotics.

That work, Dr. Hendrix said, actually revives a concept from the early days of phage science. In the 1920s, French Canadian microbiologist Felix D'Herelle traveled around the world curing cholera epidemics by identifying the phages associated with different cholera strains and using them to stop the deadly infections.

The use of phages to fight bacterial disease fell out of favor after the widespread adoption of penicillin during World War II, but it is getting a second look now because so many bacteria have figured out how to thwart antibiotics.

In 2006, the Food and Drug Administration approved a solution made up of several bacteriophages that can be sprayed onto meat and poultry products to kill listeria bacteria, which can cause lethal food poisoning.

In all of these cases, Dr. Hendrix said, phages are effective partly because they have incredibly well-honed machinery for getting inside bacterial cells.

One type of phage, for instance, has tail fibers that anchor the virus to the surface of the bacterium. A sheath around the body of the phage then twists so that it drills through the surface of the bacterium with the help of a destructive enzyme, and then injects its DNA into the cell.

Dr. Hendrix grew up in the San Francisco Bay area, and as a high school student, he had two loves -- science and the clarinet. His clarinet teacher was a scientist who had gone to Stanford University, "and he said to me, 'Well, if you go to Stanford you can learn to do something that is useful in industry like I do and you'll earn $20,000 a year. If you go to Cal Tech, you'll probably only earn about half that much, but it's more exciting science."

"And I thought, '$10,000 -- wow!' so I went to Cal Tech."

He started studying bacteriophages because one of the California Institute of Technology professors was Max Delbruck, who won a Nobel Prize for research in that field, and after doctoral work at Harvard and postdoctoral study at Stanford, he was hired by Pitt in 1973.

In January, Dr. Hendrix received a National Academy of Sciences award for scientific reviewing, largely in recognition of his extensive writing about bacteriophages in research journals.

In the PowerPoint presentation he uses to introduce people to the organisms, Dr. Hendrix often shows a picture of William Shakespeare next to the phrase, "All the world's a phage."

"That's what we believe Shakespeare actually wrote originally in 'As You Like It," he said drily, "but he had to change it after the first couple of performances because the audience just didn't get it."