We are swimming in a sea of viruses. A hundred times smaller than bacteria, these tiny things are little more than stripped-down packets of genetic material with some protein padding. By strict definition, they aren’t even alive.

But viruses are robust and promiscuous in their ability to invade organisms and hijack cellular machinery in order to replicate. The latest virus to seize the country’s attention — and create a run on bug spray — is the mosquito-­borne West Nile virus, which usually has little effect on its human hosts but can sometimes be a killer.

The Centers for Disease Control and Prevention reported Wednesday that this year’s West Nile epidemic is on track to be the deadliest since the disease first showed up in New York City in 1999, perhaps inside a stowaway mosquito on a transatlantic jetliner. There have been 2,636 officially reported cases nationally and 118 deaths, including two in Maryland and one each in Virginia and the District.

People get the virus from mosquitoes that have bitten infected birds. Most people don’t become sick, but some have a mild fever. One out of 150 develops serious symptoms, such as brain inflammation or polio-like paralysis of the arms or legs. A small number die.

The outbreaks of so many viruses in recent weeks, years and decades — including hantavirus, swine flu, bird flu, SARS, ebola and the great global scourge of HIV — raise an obvious question: Are we seeing an epidemic of viral epidemics?

The experts give a complicated, nuanced answer: yes and no. The bottom line is that virologists are hardly in a panic.

“I think it would be over-exaggeration to think that there are millions of viruses ready to jump on us and bring us back to the 14th century,” says Anthony Fauci, director of the infectious-disease center at the National Institutes of Health. “That would be looking over a ledge that isn’t there.”

But Fauci is hardly sanguine — and he’s the first to say you should use insect repellent before gardening in a mosquito-infested yard.

Lyle Petersen, director of the CDC’s division of vector-borne diseases, said this year’s West Nile season is on pace for a record number of severe infections, such as brain inflammation. These infections are considered the best indicator of the epidemic’s scope because they are most consistently reported to health authorities. Most people who are bitten by infected mosquitoes don’t develop symptoms, and their cases are not reported.

Meanwhile, thousands of Yosemite National Park visitors have been warned that they may have been exposed this summer to rodent-borne hantavirus. Of the eight people known to have contracted the virus, three have died.

The appearance of another rare but potentially deadly mosquito-borne virus, one that causes Eastern equine encephalitis, has spurred Massachusetts officials to ask residents in some communities to cancel evening outdoor events until the first hard frost. And two men in northwest Missouri were hospitalized in 2009 with a virus never before seen and possibly carried by ticks. Scientists named it the Heartland virus, after the hospital where it was identified.

The broader picture is one of threats and triumphs. Viruses evolve — but so do the medical techniques that can identify and stop them, particularly in highly developed countries with resources to monitor disease outbreaks.

Ancient threat, big target

Viruses have been part of the planet since long before humans appeared on the scene. They can infect plants or animals. When Tom Ksiazek, a virologist at the University of Texas Medical Branch in Galveston, is asked whether there are more viruses these days, he says, “The short answer is no, because I don’t think any of these things have been dropped off by a flying saucer recently.”

But there are now 7 billion people on the planet, collectively creating a meaty target for pathogens. We’re also an invasive species, probing exotic habitats and clearing rain forests where viruses lurk. Most dangerous viruses are “zoonotic,” finding a pathway to leap from an animal population into humans; the most notorious example is HIV, originally a disease of chimpanzees.

Five centuries of global trade and transport have effectively put the planet’s life forms in a blender. After 1492, smallpox and measles wiped out untold millions of people in the New World who lacked immunity to those Old World diseases. Smallpox has been eradicated through relentless vaccination and monitoring, and polio is almost gone, but there are countless other viruses that exploit the globalized economy to travel the world, perhaps inside an insect in a soggy shipping container.

Scientists can’t quite decide whether viruses should be considered alive. They aren’t like bacteria, which cause tuberculosis, strep pneumonia, E. coli intestinal infections and hundreds of other ailments. Bacteria are one-cell organisms that can repair themselves, reproduce themselves, defend themselves. They have a metabolism, a self-sustaining chemical interaction with the world. That’s also true of more-complicated microbes, such as the amoebas that cause diarrhea and the protozoa that cause malaria.

Viruses do none of those things. They have a few genes, providing instructions, and a few molecules of machinery, the equivalent of a pull starter on a chain saw, all packaged in a tough envelope. They sit, do nothing, eat nothing, don’t reproduce and wait for something living to come along. When that something is the right type of cell — viruses are choosy — they can invade, spilling the directions and the starter cord into the cell.

They use the cell’s machinery to make copies of themselves, and then they take some of the cell’s membrane — like stealing coats from the hall closet — to wrap the new crop of viruses in before they head out to look for new targets.

This parasitic lifestyle creates a challenge for anyone trying to create an antiviral drug. Antibiotics work on bacteria — living organisms. But you can’t easily target a virus without killing the cell that it has hijacked.

Better than treating viral infections is preventing them. The first successful vaccine (against smallpox in 1796) and campaigns against other scourges (such as polio, measles, mumps and hepatitis B) all had viruses as their targets.

There are clinical trials underway for a West Nile vaccine, though it remains to be seen whether companies will find it profitable to develop a vaccine for a disease that is relatively rare and usually benign. Once you get West Nile, you have immunity for life.

‘Slips through the cracks’

West Nile is not a global health problem like malaria, HIV and schistosomiasis, so big donors aren’t likely to give lots of money for developing and testing a vaccine. And it’s not a “biodefense threat” like smallpox and anthrax, so the federal government isn’t willing to underwrite a long and expensive vaccine program.

“A disease like that just sort of slips through the cracks,” said Peter J. Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston and president of the Sabin Vaccine Institute.

So for now, “put on mosquito repellent” is about all that public health officials can offer as a West Nile preventive.

Still, in the past, if you came down with a fever or a mysterious cough and visited a doctor, “the physician would say to you, ‘It’s probably a virus,’ ” said W. Ian Lipkin, director of the Center for Infection & Immunity at Columbia University’s Mailman School of Public Health. Now, virologists have tools to identify viruses down to the genetic level. Viruses are more likely to have a name.

The new Heartland virus was found when researchers were looking for the organism that causes Rocky Mountain spotted fever, which is transmitted by ticks. It hasn’t been proved yet that this new virus is tick-borne, but from a public health point of view, the message is the same: Wear long-sleeved clothing when you go outside.

The hantavirus associated with the Yosemite outbreak is the same virus first identified in 1993 in the Four Corners area of New Mexico. Scientists believe that earlier outbreak was triggered by heavy spring rains that led to a bumper crop of pinion nuts, which allowed deer mice, which carry the virus, to proliferate.

“In the evolution of the world there has always been emerging and reemerging infectious diseases,” Fauci says. “Some are threats, some are curiosities, but viruses are continuing to emerge. And that’s going to be that way essentially forever.”

Katherine Spindler, a virologist at the University of Michigan, says: “It’s like with any threat — you have to use the right precautions. . . . And then you have to live your life.”