The answer to this is that Apollo is the god who is plaguing the Greek army. The reason he is doing this is because Agamemnon has taken as a prize a woman (Chryseis) who is the daughter of Chryses, who is a priest of Apollo.
Because Apollo sends all these diseases and such to the Greek army, Agamemnon agrees to give the woman back. However, he forces Achilles to give up Briseis, who is Achilles' own prize. Agamemnon takes Briseis as a replacement for the woman he had to give up. This leads Achilles, famously, to sulk in his tent.