(CNN) -- California health officials say there's an increase in cases of whooping cough, which is believed to have killed five people in the past few months.
The disease peaks in August and September, according to Al Lundeen, a spokesman for the state Department of Health. "We have three times as many cases as expected for this time of the year."
At least two infants are among those killed, officials said.
Lundeen said the disease known as pertussis comes around every five years.
The last peak occurred in 2005 when California reported 3,182 cases, with 574 hospitalizations and seven deaths, officials said.
Earlier in the year, state officials said twice as many cases had been reported in the first quarter compared with the same period last year.
Pertussis is the most common of vaccine-preventable diseases. Although most children receive five doses of vaccine (DTaP) before kindergarten, it does not provide lifetime immunity.
Most children are susceptible again by middle-school age, officials said.
"Because pertussis vaccination does not begin until two months of age and infants are not fully immune until after six months of age, the only way to protect young infants is to do everything we can to minimize their exposure," said Mark Horton, director of the California Department of Public Health. "This means ensuring that their parents and caregivers are immunized."
Studies have shown that half the infants are infected by their parents, particularly mothers. Vaccinating caregivers for pertussis before birth of the child is the best way to protect infants.
In addition to the five doses of vaccine before kindergarten, a booster dose is recommended between ages 11 and 18, and for people in close contact with infants.
Women may be vaccinated before pregnancy, during pregnancy, or after giving birth, officials said. Fathers may be vaccinated at any time, but preferably before the birth of their baby.
Symptoms of the disease include a runny nose and rapid coughing fits that end with a whooping sound.