If by parasites you mean the spores on the levels floodgate,cortana and halo no they do not count as kills so sorry.
an overkill is 4 kills
Yes.
If You Do Not Count Halo Wars,Halo 3:ODST And Halo Reach As The Series No Information About If Halo 8 Will Come Out Or Not.
Shoot and kill people.
If you count Halo 3: Recon, but you don't play as the master chief on that.
he lands on earth kills prophet saves cortana and defeats the flood.plus he kills the oracle
no but there are flood [ parasites that take over people and infect them and practicly turns them in to zombies]
There is none, it only goes to 25.
The first skull on halo 3 is a silver skull. Silver skulls do not award any achievements.
THere is no spree for the set number 13. The two closest are killing Frenzy (10 kills) and running riot (15 kills).
You win games to earn EXP.
Nothing does, it's the maximum amount of kills in a row you can get so far. They might change that in Halo Reach tho.