Do strange parts apply retroactively?

For example, let's say I have a Strange Scattergun with no parts attached and no kills.

I kill 5 medics, then apply a 'Medics Killed' part to the Strange Scattergun.

Then, I kill 5 more medics.

What stat will be displayed? 5 medic kills, or 10 medic kills?

Or another way of phrasing it - will strange parts reflect statistics you accrued before applying the part? Or does it only start counting those statistics after it's been applied?

The answer is a simple NO. Strange parts only begin to count the statistics after they are applied. Previous stats are not accounted for.

When applied, it will not count any previous stats.

Reference: TF2 Wiki