last year, we're seeing about a 300% increase in rsv cases. just a very striking people, and we don't know if we've peaked yet. >> reporter: respiratory instishial virus or rsv is the most serious in infants, young children, elderly people, and those with weakened immune systems. that cough led this california mother to seek treatment for her2-month-old daughter. >> we noticed that she was coughing a lot an had a really runny nose. we took her in to the doctor and had her tested and, sure enough, she had rsv. >> reporter: cases have more than doubled in the past month and are more than five times higher than last year according to the cdc. >> we're at capacity on many days, and it's one patient in, one patient out. >> reporter: across the country, pediatric hospitals are filling