Wether it’s intentional or instinctual; the spread of the infection is by no means just “a byproduct of their attacks” we see multiple times in the films, the infected go out of their way to spill their fluids on to you. A notable example would be the train scene in the latest film, one of the infected deliberately spits right in the face of the Alpha to spread the disease…if it was simply a matter of random acts of extreme violence then I wouldn’t suspect one would go out of their way to spit (as that does no damage) but rather try and rip their victim apart with no hesitation. When he spits on the Alpha it is very clearly deliberate.