How he contacted HIV through Watermelon

These young boy on that particular day walked up to a roadside mallam selling fruit to buy Watermelon. Some minutes later, he ate the Watermelon not washing it. Few days later, he fell extremely ill. At the hospital he was also tested for  HIV and the result was Positive. These boy in question is not promiscuous as he is  still young and the question is How did he contact HIV ? After series of investigation, it was traced back to the mallam who sold Watermelon to him. According to eye witnesses around, the said mallam in process of cutting the fruits mistakenly cut his hand and a drop of his blood dropped on the watermelon which the boy bought and ate without washing. That caused the boys misfortune. The mallam did not know he has being infected with the Virus. Warning -  Wash your fruits before Eating.