Health authorities say the respiratory syncytial virus is making young children sick, but no more than usual.
RSV infects the lungs and breathing passages. It has the same symptoms as a cold, like coughing, sneezing and a fever.
Doctor Ann Thomas of the Oregon Health Authority says it's hard to say how many people may be suffering.
"It's not a reportable disease in Oregon. Nor is it reportable in most states," she says. "We do have a voluntary reporting system set up with the 20-plus labs in Oregon who do the testing. And each week they tell us how many RSV tests they've performed and what percentage of those are positive. And when we hit 10 percent, we say that RSV season is here."
The number of positive tests hit 10 percent back in December; 22 percent are now coming back positive, which Thomas says is not unusual for this time of year.
RSV is very common in early childhood. By two years of age, most kids will have had it. Children most at risk are premature, or have congenital heart disease or chronic lung conditions.