Canada boasts one of the best healthcare systems in the world, with great effectiveness, affordability, and accessibility. The health situation in Canada is very stable, and tourists are highly unlikely to face any issues with care or illness that aren't present in other industrialised countries.
Although health care is free for residents, it is not free for visitors. Make sure you have good insurance before you leave, especially if you're hitting the slopes.
As a side note, indoor smoking is typically banned in public places and near entrances of buildings, as well as bus shelters and patios.
You don't need any immunisations to visit Canada on your gap year.
Luckily, safety in Canada is rarely a major issue. Using common sense and staying out of potentially dangerous areas will typically help you avoid any crime or violence. Violent areas with drug battles and gangs are typically contained and crimes rarely befall uninvolved tourists. Crime rates in Canada are typically much lower than those in similarly sized and industrialised areas in North America. Crimes such as shootings and violence are typically higher in the western areas than the east, but even then tourists rarely have anything to worry about while traveling through Canada. Although there have been shootings in public areas like malls and urban centers, they are extremely rare.