Californians are rushing to get themselves inoculated against a whooping cough epidemic which has claimed the lives of several babies.
The number of people with the infection -- named after the strained coughing sound sufferers make -- has reached a 55-year high, including nine infants who have died, eight of them Hispanics.
"In addition to children, who are the most vulnerable, those who care for or are in contact with children should also be immunized," said pharmacist Walgreens, which is offering inoculation at 150 of its stores.
Up to mid-September there were just over 4,000 confirmed, probable or suspected cases of the illness -- also known as pertussis -- the most reported since 1955 when 4,949 cases were recorded in the whole year.
Whooping cough is a highly contagious bacterial infection affecting the lungs which is usually relatively mild in adults but has a high mortality rate among children.
"In addition to children, who are the most vulnerable, those who care for or are in contact with children should also be immunized," added Bill Hose, California market vice president for Walgreens.
Australia's foreign affairs department on Thursday warned travelers to the United States to watch out for the outbreak, which has also been reported in a number of other US states.
The department maintained its overall advice at "exercise caution," the second of a five-level warning system.