The Cassandra Crossing (1976)
Pssst, want to check out The Cassandra Crossing in our new look?
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Outbreak (1995) has a remarkably similar plot to this with the exception of the African monkey part, which is due to Outbreak's origin as being based on the 1994 novel The Hot Zone. Definitely seems like the writers lifted the stronger parts of this movie for their script.
However of the two I'd say Cassandra Crossing is the weaker movie, watered down by its "Murder on the Orient Express" style trainbound cast of characters that dilutes its ability to move past archetypal depictions of most of the characters.
to see which of your friends have seen this movie!