Why couldn't Philip beat the hell of the Infected?

[Thaliur]

Dude, how in the world would a virus alter someone so much?

It's an intelligent alien virus, I guess.
Even our "local" dumb virus types are able to alter human DNA. A common flu can often be diagnosed by DNA analysis generations after it was cured.

I don't know how exactly the Tuurngait virus manages to alter the humans that quickly, but if something is capable of such extensive physical changes (the weird umbilical cord thing for example) I wouldn't be surprised if reinforcing the subject's ski is the first thing it does.