As a breastfeeding advocate, let me step in here.
All milks, including human and cow, have live immune cells. That's a major benefit of breastfeeding. It innoculates the baby against any germs that the mom has encountered. The live cells are the same ones that gobble up invading germs and turn into pus. Technically, they are not pus unless they have done battle with an active, virulent germ. Unitl then, they are white blood cells.
Pasteurization kills all the cells that occur in the milk. So, unless you are drinking raw milk (which is hard to come by in the US), the cells are no longer alive. If the milk you are drinking came from an infected cow, as PP stated, then yes there may be some pus in the milk, thought both the WBCs and bacteria will be killed by pasteurization.
I have heard this arguement stated as if milk is just cow pus. If that was true, baby calves would be horribly malnurished, but impervious to germs.