Complex sentence: The success of the virus was found to be higher in extreme heat, because it appears there is another calicivirus in the colder, wetter areas of Australia, and that this virus was immunising rabbits against the more virulent form.
Split into simpler sentences: The success of the virus was found to be higher in extreme heat. This was because it appears there is another calicivirus in the colder, wetter areas of Australia, and that this virus was immunising rabbits against the more virulent form.