That's pretty much the setup for the horror that ensues. I won't give too much more of the specifics away. For a full synopsis, check out the Wikipedia article:
The Thaw on Wikipedia
The movie itself is just slightly better than average for a horror film. One of the antagonists of the film, a student who is infected with the parasite, kind of destroys the plot. As a case of bad writing, the character takes control of the group with a rifle which he hangs over his shoulder which is in no way threatening (if myself or a group with at least one or two brave people were involved that kid would have been knocked out, tied up, and gagged rather quickly). There is a rather enjoyable amputation scene, but otherwise the film is fairly predictable and slow.
The parasite that is killing the characters of the film is a vertebrate organism which functions rather like a parasitic insect. The male of the species causes a sore on a host which attracts females. The female will lay eggs within the sore which pass through an egg stage before hatching and crawling into their host for nutrition. The infected hosts' bodies are eaten inside-out by the insects. This is a common strategy for life on our planet, especially amongst insects. I think the idea of such a destructive parasite laying dormant in the flesh of a mammoth is probably why I enjoyed the film. It is very possible that there are many forms of life which we have no remaining record of which may have had peculiar or unusual metabolic strategies or life cycles. Also, it's fun to speculate how a biosphere similar to ours would react to the release of an organism or community which had not been active for a time that is long enough to allow the incoming organisms to overwhelm all of the surrounding biosphere, just like an invasive species taking hold of an environment.
I would definitely recommend watching this film if you enjoy sci-fi and horror, though it's definitely not great by any means.