Are there habits that make bronchiolitis more likely?

Yes! Bronchiolitis, a viral infection of the bronchioles (the smallest airways), usually occurs in patients under 2 years old and is most common in babies 3-6 months old. One out of every 9 babies catches it before 1 year old. It's slightly more common in boys, and exposure to secondhand smoke definitely increases the risk. Daycare attendance increases the risk simply by increasing exposure to germs.