24 (2001-2010)
In the final moments of 24‘s season 2, U.S. President Palmer greets the public and is rendered unconscious by a handshake-transmitted virus. The assailant, Mandy, was a minor character in the first season, making her last-second appearance a real callback twist. Who is Mandy working for? And why? And why not kill the President instead of just making him real light-headed? All this and more…not explained in season 3!