Oswald did it! He was part of a CIA plot to infect Castro with cancer back in the 1960s.
Flash forward to 2012, Ernesto Guevara, from his luxurious underground bunker/t-shirt emporium becomes worried that Potato Head Hugo will cut into Ernesto's t-shirt sales. He confers with his great friend, Fidel the Death man, who pulls out a phial of the cancer serum Oswald bungled and let fall into Castro's hands, which was the real reason Jack Ruby took him out, and let's Ernesto poison Potato Head with it.
Of course, all this happened in December, they decided to hold off announcing it until March 5 for symbolic reasons, namely being the day that Stalin was poisoned.
Symbolism is of the utmost importance to socialists because they are incapable of integrated thinking.