HOUSTON -- Houston hospitals have noticed the comeback of an illness once thought to have disappeared through years of vaccination, News2Houston reported Wednesday. More doctors across the country are diagnosing children and adults with pertussis, more commonly known as whooping cough.

Although the illness can be dangerous for children, the Texas Department of Health reported that only 78 percent of children are vaccinated against it.

Since whooping cough is very contagious, early detection is key to preventing infection and beginning treatment.

"I was very concerned about all the people I had infected," said Lisa Holtin, a patient.

She started feeling the symptoms in April.

"I was really struggling with breathing," Holtin said. "You start coughing, and you can't stop."

At first, she was diagnosed with bronchitis, then asthma. Dr. Mila McManus, with Alden Health Center in The Woodlands, was able to give the correct assessment. She concluded that Holtin had pertussis.

"It took several weeks for me and several visits with her until I witnessed her having coughing fits that made me suspicious," McManus said.

A series of coughing that sound like a baby cough is one of the telltale signs of whooping cough.

"The whoop is when they're breathing in. They've coughed out all their air," McManus explained. "They try to breath in and make this loud noise because they're breathing against the resistance of their vocal chords."

At Texas Children's Hospital, doctors have seen at least one case a week. It's especially dangerous in children who can literally stop breathing.

"When you're coughing -- just breathing out, you're not able to breathe in and oxygenate yourself," McManus said. "So, these babies often turn blue, too."

Pertussis is the only vaccine-preventable childhood illness that has continued to rise over the last 20 years.

Symptoms include the signature whoop sound during a coughing fit, thick mucus, and vomiting. In some cases the lips and nails can turn blue due to lack of oxygen. Usually patients are exhausted from incessant coughing.

"You're coughing so much that your whole chest will hurt," Holtin warns.

Since Holtin's case, McManus has diagnosed six more cases of the whooping cough. She's concerned others may be misdiagnosed.

"It's very rare. A lot of people think it's been irradiated since we started vaccinating our children," she said.

The government will decide if it's time to add more booster shots for children and adults, since doctors have noticed that the vaccine can wear off in adulthood.

Whooping cough can be treated with antibiotics if caught early. Without an early diagnosis, the illness can take weeks, or even months, to run its course, according to doctors.