Does the West Nile outbreak signal an epidemic of viral epidemics? Yes and no.

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. WashingtonPost