Alex inherited not only his father's throne, but the best equipped, trained and organized army of that time, or indeed before or long after that time.

Legend says Alex was born on the day the Temple of Artemis burned to the ground. He inherited the throne at 21 and had conquered the known world (mostly Persia) by the time of his death 12 years later. His greatest gift was to make the conquered peoples happy so that they would not rebel later, though eventually this angered his own forces. Alex liked to found cities and name them Alexandria, the greatest of these was in Egypt and featured the Lighthouse of Alexandria.

For the curious I recommend The Generalship of Alexander the Great by J.F.C. Fuller. BTW, Fuller cites malaria as the cause of death, too.