A 17-year-old student at Petaluma High School was hospitalized over the weekend with a highly contagious blood infection caused by a bacteria that also can cause meningitis.

School officials sent a warning to parents late Monday with automated phone calls that those who may have had close contact with Christopher Pozzi-Swain, a senior, should alert health officials.

Pozzi-Swain suddenly became ill Saturday and collapsed with exhaustion, said Dr. Lynn Silver Chalfin, Sonoma County's public health officer. He was hospitalized that night.

The teen's condition had greatly improved Tuesday, although he was still in serious condition at a hospital, Silver Chalfin said.

"Chris is feeling much better," Petaluma High Principal David Stirrat said.

Doctors determined Pozzi-Swain was stricken with a blood infection caused by the bacteria Neisseria meningitidis, a bug spread through saliva or phlegm.

Called meningococcal disease, the infection causes symptoms including a high fever, rash and stiff neck. The disease is treated with antibiotics. If untreated, meningococcal disease causes death in about 50 percent of cases, according to the health department.

The bacteria is part of a group that can lead to meningitis, an infection of the membrane surrounding the brain and spinal cord.

Meningococcal disease is spread through close contact, such as kissing, sharing drinks or utensils or "getting coughed on," Silver Chalfin said. A person who merely passed an infected individual in the hall or sat in the same classroom is unlikely to have been exposed.

But for those who may have been exposed, a dose of antibiotics can prevent the bacteria from causing an infection, she said.

County health staff have been aggressively contacting all who came into close contact with Pozzi-Swain, Silver Chalfin said.

Family and several friends already have received treatment. That includes students at Casa Grande High School, where Pozzi-Swain used to go and still has friends, Stirrat said.

Health officials had found no other confirmed cases of meningococcal disease.

The day after Pozzi-Swain became ill, a Casa Grande student was hospitalized with similar symptoms.

However as of Tuesday doctors don't suspect a blood infection, Silver Chalfin said.

"He had a positive flu test, although we're still treating him out of caution, but most likely it is not a second case," Silver Chalfin said.

Pozzi-Swain's condition had greatly improved Tuesday, said Dave Rose, director of student services with Petaluma city schools.

"They're still running tests, and they're being cautious, but his family and friends are able to visit. He's doing better," Rose said.

Anyone who may have been exposed to the bacteria between Feb. 9 and Feb. 16 can call the Department of Health Services at 565-4567.

You can reach Staff WriterJulie Johnson at 521-5220 or julie.johnson@pressdemocrat.com. On Twitter @jjpressdem.