A tenth baby has died in the worst outbreak of whooping cough for decades, Health Protection Agency figures for England and Wales show.
Cases continue to soar with 1,322 more people infected in September, bringing the total to 6,121 this year.
Newborn babies are most at risk of death from the disease.
Last month a UK-wide campaign was launched to vaccinate pregnant women in order to pass protection on to their child while it is still in the womb.
There have been more than 1,000 cases of whooping cough in Scotland and nearly 200 cases in Northern Ireland, but no reported deaths.
The infection can stop a baby breathing or lead to pneumonia, brain damage, weight loss and death.
However, newborns are too young to be protected by routine vaccination, which starts at two months of age.
So women who are between 28 and 38 weeks pregnant are now offered a whooping cough vaccine. The idea is to boost the mother's immunity, which is passed on to the child.
Dr Mary Ramsay, the head of immunisation at the Health Protection Agency, said: "We have been very concerned about the continuing increase in whooping cough cases and related deaths.
"All parents should ensure their children are vaccinated against whooping cough on time, even babies of women who've had the vaccine in pregnancy - this is to continue their baby's protection through childhood.
"Parents should also be alert to the signs and symptoms of whooping cough - which include severe coughing fits accompanied by the characteristic "whoop" sound in young children but as a prolonged cough in older children or adults.
"It is also advisable to keep babies away from older siblings or adults who have the infection."
Dr David Elliman, who's an immunisation expert at the Royal College of Paediatrics and Child Health, encouraged mothers to have the vaccine.
"When people have looked at where young babies get whooping cough, how they acquire it, common sense would tell you it's probably someone within the family and in fact, in practice it turns out to be the mother, which again is what you'd expect.
"So this is the logic behind giving the vaccine to mothers.
Routine vaccination was introduced in 1957. Before then cases could affect in excess of 100,000 people and kill 300 in a single year.
There are surges in whooping cough cases every three to four years. The current outbreak started at the end of 2011, but cases are already seven times higher than the last outbreak in 2008.
Health experts do not know why the outbreak is so large this year, especially as vaccination for whooping cough is at record levels.
One theory is that the bacterium which causes the infection, Bordetella pertussis, has mutated.
Another idea is that tight control of whooping cough is part of the problem. Repeated infections of whooping cough used to naturally boost people's immune systems.
However, after years of low levels of whooping cough the whole population may be more vulnerable to the infection.