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With four provinces now battling whooping cough outbreaks and the U.S. on track for its worst year for the illness in five decades, public health officials are urging all adults to make sure they are fully vaccinated – especially if they plan to spend time near infants.

New Brunswick has seen more than 1,000 confirmed whooping cough cases this year, and Alberta Health Services has seen dozens of cases of its own in southern Alberta.

That’s where one-month-old Harper Whitehead died last month from complications caused by the bacterial infection, which is technically called pertussis.

There are also outbreaks in British Columbia’s Fraser Valley and parts of southwestern Ontario. The U.S, meanwhile has seen nearly 18,000 cases, and nine child deaths.

So what’s fuelling these outbreaks? A few things, it seems.

Epidemiologists note that whooping cough is typically a cyclical illness that tends to peak every two to five years. But decreased immunity from improperly vaccinated adults may be fuelling much of the spread.

Whooping cough outbreaks are often caused not by people who have refused the whooping cough vaccine, but by those who don’t realize their vaccinations have worn off.

Studies have shown that most children pick up the illness from someone in their home, many of whom didn’t realize they were carrying the infection.

Infants are most at risk from the illness, which can lead to brain swelling and death. That’s why health authorities in various parts of the country are urging all adults spending time with babies to ensure they are fully protected against the illness, which is spread through cough droplets.

Most adults in Canada have received pertussis vaccines when they were younger. But the vaccine has a notorious ability to  wane Adults with worn-off immunity can act as "reservoirs" of disease, becoming unknowingly infected and then passing the illness on to a baby or child.

Infectious diseases experts note there are inherent challenges in generating immunity to whooping cough, because of the way the bacteria invade the immune system. Unlike the measles vaccine, for example, which is very effective, it’s often difficult for the immune system to generate immunity to whooping cough.

That’s part of the reason why kids need only two doses of the measles vaccine, but they need five doses of the whooping cough vaccine -- plus a booster in the teen years and another booster in adulthood.

Canada's National Advisory Committee on Immunization (NACI) recommends that infants be immunized against pertussis at two, four, and six months of age with Daptacel (DTaP, a combination of tetanus, diphtheria and pertussis).

They should then get another dose at 18 months of age and again between four and six years of age. Then, when kids are between 14 and 16 years of age, they should get a booster of Adacel, which is another formulation of the three vaccines, called Tdap.

Adults should get another Tdap booster at least once more.

Because so many doses are needed to build immunity, babies under six months of age are at high risk of serious or fatal illness, because they don't have full immunity even if they’ve begun to get the vaccines.

That’s why women planning to get pregnant and their partners should ensure they have had the adult whooping cough booster, so that they don't infect their babies before they're fully protected.

Patients who don't know if they're up to date should talk to their family doctors, who can often figure it out based on age and where they grew up.

New Brunswick’s health department says those who have not been vaccinated against whooping cough during the last five years and are in close, regular contact with children younger should contact their doctors and ask for the vaccine.

The most common reactions to the vaccine include soreness, redness and/or swelling at the site where the vaccine was given, and mild fever.