When a Divine punishment was mandated, G-d gave David the choice of a famine, war or pestilence. David told the prophet Gad that he preferred to fall into G-d’s hand than man’s hand. Accordingly, G-d sent the pestilence. We understand how David’s response eliminated the option of war but how did it exclude the famine? Rashi explains that in a famine, rich people can manipulate things to their advantage. Only the effects of a pestilence would be completely up to G-d. (This verse is the first line of Tachanun.)