I have a Pokemon with magic guard and is holding a Life Orb. It knows the move Infestation. I know that infestation can last 4-5 turns. Will every one of the continuous hits by Infestation be boosted by the Life Orb or just the initial attack?
On the turn Infestation is actually used as a move, it will be boosted because it's treated as a normal attacking move with base 20 power.
However, after actually attacking, Infestation does fixed residual damage - 1/8 of max health - and therefore is not eligible for Life Orb boost.
Bulbapedia - Infestation