Dean was born in 1948.
I don't have any history texts handy as a reminder, but I did find a list of presidents since 1948:
Truman: Used atomic weapons, Korean war
Ike: Opposed government funding of polio vaccinations because it would be "socialized medicine". (I'm not entirely sure about that one; it may have been a different president, but I'm fairly confident it was around Eisenhower's time...)
JFK: Came very close to pushing brinkmanship too far on multiple occasions
LBJ: Something about a war....
Nixon: Various "dirty tricks" and disgracing the presidency
Reagan: Used military spending to hasted Soviet collapse; gambled with provoking the Soviet Union into starting a third land war in Europe. Won, but was extremely dangerous.
Clinton: Ineffective against al qeada.
Public health is-- for the moment-- codespeak for mad cow disease. Clearly, polio is far more dangerous than mad cow disease, and the policy of allowing the disease to go unvaccinated was far more dangerous than one imported cow getting sick.
Now, you probably disagree with one or more of these; they're more random incidents that come to mind than fleshed out reasons. But I find it astonishingly ahistorical to call Bush "the most dangerous". Even if you don't like him, nothing he's done-- or even TRIED to do-- can approach the scale of the presidents in the latter half of the twentieth century. It's not even good rhetoric, because the electorate skews towards older demographics who remember previous administrations...