Three students who went to Fort Collins public schools have whooping cough and Larimer County health department officials worry that they may have infected others.

While those students are taking antibiotics and are no longer contagious there may be others out there who don't realize they have whooping cough, Larimer County health officials said. A number of family members have signs of the infection and are being investigated and treated.

"Because the symptoms of pertussis are often mistaken for other kinds of respiratory infections, people may not seek medical care and may continue to spread the disease which is particularly serious for infants under one," said Ann Watson, with the Larimer County Department of Health and Environment.

The illness starts with symptoms similar to a common cold, including a runny nose, mild fever and sneezing.

Symptoms of whooping cough may be mistaken for other illnesses particularly in adults, teenagers, and vaccinated children because their symptoms are milder, may mimic bronchitis or asthma, and generally do not include the "whooping" sound, Watson said. The "whooping" sound is often also absent in young infants under 6 months of age.

The respiratory infection can easily spread through the air from a sick person during talking, coughing or sneezing.

Larimer county had eight reported cases last year and three confirmed cases so far this year. Between January and March 21, a total of 88 cases have been reported in Colorado, 45 of them in Boulder County.

Pertussis can go on to cause pneumonia, seizures, and occasionally death. It is especially serious in young infants, with a fatality rate of 1.3 percent in children less than one month old.

Doctors recommend that children and adults who have had a prolonged, persistent cough (lasting over 14 days), or a cough accompanied by whoop and vomiting be examined by their family doctor.

Young infants under 6 months old may not have the whooping sounding cough. They may appear blue and have difficulty getting their breath after coughing. Infected persons should be excluded from work or school until they have been on antibiotics for five days.