A fatal--and rare--infection that killed a San Diego boy reinforces an important fact about infectious diseases: they are often zoonotic. A zoonotic infection is one that arises from a species jump. HIV is a zoonosis from chimpanzees, rabies is a zoonosis from dogs, and in this case, Rat Bite Fever is a zoonosis from rats.
Rat Bite Fever is a rare illness caused by a bacterium known as Streptobacillus monilliformis and it is fatal in about 20% of cases. Plain penicillin is all that is needed to abort the infection, if the disease is recognized.
Rats carry the bacteria without symptoms so it's not obvious if a rat is infected or not. As such, caution is advised when handling rats (use gloves), especially in those with an immunocompromising condition.
This fatal case has sparked a lawsuit against Petco but I can't imagine they are unequivocally to blame for this tragic occurrence.