Analysis - Anthony Zurcher, BBC News, Washington
George HW Bush once vomited on a Japanese prime minister. His son fainted in the White House after nearly choking on a pretzel. Franklin Roosevelt hid his serious health conditions, and John F Kennedy never spoke of his debilitating back condition.
The difference between these men and Hillary Clinton, however, is that her "overheating" episode comes in the home stretch of a presidential campaign where she seeks to tie Ronald Reagan as the oldest person ever elected to a first term as president.
Then there are the conspiracy theories about her health - some of them advanced by top Trump campaign surrogates - which will only become more prominent, particularly after Mrs Clinton left her press pool in the dark for more than two hours on Sunday.
Age has been a factor in several recent presidential elections. Both John McCain in 2008 and Bob Dole in 1996 would have easily shattered Reagan's record. Donald Trump, at age 70, has his own health questions to address.
Sunday's episode may end up being simply an annoying distraction for the Democrats. Now, however, people who would otherwise have shrugged off Mrs Clinton's health rumours will be watching her activity more closely.
Coverup is ongoing. The press knows she's "not right." All that's changed is the addition of a new piece of public evidence, desspite best efforts to prevent such evidence from being created.
The system can tolerate millions of people knowing the truth. Millions of people know or believe that dual citizens aren't qualified to be president, for example.