Anything that has to be spread to "The Masses" through marketing, propaganda or public education campaigns need a simple message to work. Simple, clear, strong messages with no room for debate. Just say no. Just Do it. The Real Thing.
Exaggeration is absolutely essential if you want to get people to react. You cannot rely on reality to provide the necessary frightening statistics.
So for example, almost everyone (including smokers) overestimates the risk of lung cancer caused by smoking by an order of magnitude. The danger of second hand smoke by even more. Cigarettes are unhealthy, but people are too irrational to react to that. In order to get a reaction, the dangers must be multiplied.
Another good example is HIV transmission. Most people assume the transmission rate is close to 100%. Someone who has had sex with a HIV+ partner and survives dodged a bullet. In reality, transmission rates after a single exposure are very low. It depends on the sex act and viral load but even for high risk activities like anal sex, infection rates are in the low single digits. They are estimated around 0.1% per exposure for vaginal penetrative sex.
Campaigns that made you think of how many people are 5 shags removed from you (your ex & her exes & his exes..) implied that infection rates were near 1%. People bought it. They started using condoms. The spread slowed or stopped. A 0.1% risk would not have done that.
So yeah. Marketers lie. So do propagandists. The layman public simplify. It's how shoes get sold. it's how public opinions are changes.