Alice believes sharks are the biggest problem facing humanity.
If you ask her, she'll tell you about all documented cases of shark attack. She'll tell you how it's definitely undercounted - random disappearances will be counted as drownings even if they were shark attacks. Or someone could actually drown but only because they were running away from sharks.
She'll tell you that population increases every year, mostly in warmer and so presumably more shark-infested areas, that most people live in coastal areas, so we can expect more shark attacks in the future.
She'll tell you that as people get wealthier, tourism increases, and more tourists means more possibility of shark attacks.
She'll tell you that even living far away from the coast doesn't guarantee safety, as some sharks live in rivers. And what stops sharks from swimming into canals and sewer systems really?
She'll tell you that direct sharks attacks are just start of the problem. Sharks will eat fish, causing famines or mass unemployment in fishing dependent regions. Panics caused by shark attacks might cause mass migration, and all that together increases likelihood of armed conflicts. The more you talk to her, the most every problem turns out to be related to sharks.
She'll tell you how we need global solution to shark problem. Armed guards on every beach. Shark radars. Maybe even security bars in every toilet so shark won't swim up to bite you in your most vulnerable moment. The more you talk to her, the more elaborate her solutions become.
And of course she'll happily provide links with evidence for all those claims.
Bob doesn't believe sharks are even real.