"RSV, or respiratory syncytial virus, is one of the common causes of lower tract infections in children, and one of the most common causes of hospital visits in infants and children," said pediatrician Dr. John Mangoni.
Mangoni says RSV season is definitely here. Hospitals say they've seen a dramatic increase in the past week.
A Children's Hospital Los Angeles spokeswoman says the 15 cases their doctors saw on Monday are far more than they usual they see in a week at this time.
Experts say this particular virus is especially dangerous to infants and young children born premature, have heart disease or asthma.
"RSV can cause bronchospasm, or like wheezing symptoms, and because the bronchial tubes become constricted, they can't get air into their lungs as well, and therefore their oxygenation is decreased," said Mangoni.
RSV is spread by respiratory droplets or by hand-to-hand contact. So even though it's rude, doctors say it is a good idea to tell people to wash their hands if they're anywhere near your infant.
"The virus RSV can live on your hands for about a half-hour and can live on the environmental surfaces for a couple hours," said Mangoni.
RSV can only be treated with supportive care like fluids and rest.
Mangoni says hand-washing, covering your cough and staying in when you're sick is the best way to prevent the spread.