(CNN) -- A high school varsity athlete, a sturdy guy with a health history blissfully free of blips, 18-year-old Joseph Spencer had little reason to think anything was seriously wrong when he got sick last April.
Doctors say Joseph Spencer could have died from adenovirus, a virus that usually just causes a cold.
The vomiting, chills, fever -- "It must be the flu," he thought.
Within hours, Spencer's fever was 104 degrees. Within days, he was in the intensive care unit at Providence Portland Medical Center in Oregon with full-blown pneumonia. Spencer's doctor was afraid this sturdy teenage boy was going to die.
"His lungs had filled up with water, it was hard to get oxygen into him," explains Dr. David Gilbert, an infectious disease expert and Spencer's physician at Providence. "Things got so bad, I thought we were at risk of losing him."
But as perplexing as what would make a hardy young man so sick -- so quickly -- was his diagnosis: adenovirus, the virus that usually causes nothing worse than a nasty cold.
"In the past, we considered adenovirus a 98-pound weakling," says Dr. Dean Erdman, leader of the respiratory diagnostic program at the Centers for Disease Control in Atlanta. "But adenovirus is causing severe disease and, in some cases, death in normal, healthy people."
At least 1,035 people in Oregon and a handful of other states have been infected by adenovirus so far this year. One of the largest outbreaks was at an Air Force base in Texas. Watch Dr. Sanjay Gupta explain the killer cold virus »
But by far the deadliest outbreak is in Oregon, where seven patients have died.
"Fortunately, Joe survived," says Gilbert. "Twenty percent of our patients did not survive this severe infection."
The bug causing the serious disease is called "adenovirus 14," one of the 51-odd strains of adenovirus that typically cause anything from colds to conjunctivitis and gastroenteritis. While adenovirus isn't a new bug on the block -- it was first identified in 1955 -- researchers believe it has mutated into a more virulent form, first identified in 2005.
"We were very surprised when we ran into this much more aggressive form of adenovirus, which took otherwise healthy people and put them into our intensive care unit with life-threatening pneumonia," says Gilbert.
It is important for doctors to think of adenovirus when confronted with an ill patient -- recognizing early symptoms before they become life-threatening. Infectious disease experts say shortness of breath, cough and fever are all symptoms you should not ignore.
But how big a threat is adenovirus 14 really?
Adenoviruses are ubiquitous, scrappy bugs -- they exist on everything from pens to countertops to inside our noses. They are spread through contact with a surface, or through the air we breathe. Most people won't suffer life-threatening illness if exposed to adenovirus 14, and that strain of the virus is still pretty rare, but since few people have antibodies to it, there's opportunity for a new virus to spread rapidly throughout the population.
"Adenoviruses kill people," says Gilbert, adding that when these infectious viruses do spread, they spread fast.
"We are asking physicians is to be alert, not to panic -- but be alert," says the CDC's Erdman, who stresses that influenza remains a much larger public health concern, killing and causing far more serious illness annually than adenovirus.
Experts stress that one of the most important things for doctors to recognize is whether a patient has an acute respiratory illness. It could be adenovirus, something they may not connect to severely ill patients.
As far as treatments for adenovirus 14, there aren't any -- doctors focus on managing symptoms -- but researchers are trying to determine if any antiviral drugs could be effective.
The CDC stresses that while a flu shot is a good idea and you should get one, it won't protect you against adenovirus 14. But common sense will: keeping household surfaces clean with a good virus-buster like bleach, avoiding and covering up coughs and sneezes, and of course, hand washing.
Spencer is taking life a little slower these days. He still has gaps in his memory, probably due to oxygen loss. He's made lifestyle changes, including popping vitamins as religiously as he now washes his hands: He keeps bottles of hand sanitizer in his car and on his nightstand.
Most of all, he wants people to know that adenovirus is out there, and what it can do.
"I never thought this would happen to me. You'd think it only happens to unhealthy people," he says, pausing to find the words to finish his sentence. "I always thought of myself as a healthy guy until this happened.
"People need to be aware there's a killer out there." E-mail to a friend