Just caught up with the last episode. Wow.
That was grotesquely brilliant.
So, to recap: Liam definitely killed Charlie (even though we didn't actually see it), and then in last week's flashback he told his dad that he'd 'thrown up'. So, having seen the chain of events first hand with Shelley this week we can assume that Liam cut Charlie's stomach open, vomited the frogspawn-stuff into his gut and then tried to close him up again, like Shelley did to her mum.
The mum is still alive - we saw a deliberate shot of her still breathing - and Charlie was also still alive when Liam left him, he just died later in the ambulance. So perhaps the virus/invader is trying to propagate by infecting its victims in this way.
Shelley came into first-hand contact with the mammoth remains when she investigated the lock-up a few episodes back, so we can maybe assume she was infected that way (especially as she coughed at the time) rather than an infection in the water, as suggested by last week's shot of the fluid leaking down the drain.
The bastard with the boat has made off with the tusks. A possible further outbreak of the virus there.
Ronnie is still suffering with his hand but isn't necessarily infected - unless he, or Carrie, WERE infected and have already passed the frogspawn on. Remember the scene in the tent, when Carrie woke up in the middle of the night because something (a reindeer?) was outside? Ronnie went outside for awhile, then came back and said it was nothing, but in the morning we saw a shot of blood on the ice. Maybe we'll get a flashback of Carrie, in a trance-state, going outside that night and vomiting into a reindeer's stomach (I can't believe I just typed that sentence).
Yuri, the Mad Russian, is awesome.
Anything else?
Love this show, and it's careful and deliberate flipping between crime and horror. Such a shame it's getting critically panned and hardly anyone's watching it...