Pay very close attention to your team. If you see someone doing odd things, such as not shooting at enemies when they appear, or moving rather far away from you when your walking, be on alert as this could be a sign of infection. However, do not waste a Blood Test Kit on him to see if you are correct. Just be on alert because they could mutate at any second. This also goes for you as well -- if your team sees you doing the same things, they will question your trust and may attack you.