SAN DIEGO -- In Oakwood, Ohio, schools closed for the holidays a few days early after nine students were diagnosed with whooping cough.

Whooping cough, or pertussis, is a highly contagious bacterial disease that may at first appear to be a common virus.

"It will start off with sort of flu-like symptoms with a mild runny nose, clear runny nose, that progresses to a cough and that first stage lasts about a week," Sharp Healthcare pedatrician Dr. Michael Martin said.

What follows is a serious cough that may not go away for six to eight weeks.

The introduction of a pertussis vaccine decades ago drastically reduced the number of whooping cough cases in the United States, reaching its lowest point in the 1970s.

However, now pertussis cases are on the rise.

"Last year in 2004 there were about 25,000 cases reported," Martin said.

While pertussis can affect people of any age, it is especially hard on babies.

Little babies will have this prolonged coughing spell where they'll cough 50 to 100 times in a row, not be able to breathe. At the end of that spell, they'll take a deep breath and the inhalation makes that characteristic whooping sound," Martin said.

A series of vaccinations in the first year of life can protect a child.

But parents should still be cautious if they think their child could be exposed to someone who is infected -- especially if the child is under four months old.

"If you suspect that someone has whooping cough, you'll definitely want to keep them away from all infants," Martin said.

Babies can be protected by making sure any siblings over the age of 11 have received the newly recommended pertussis booster shot.