Complex sentence: In the 1900s, a parasitologist had noticed that "Rattus macleari", a species of rat endemic at the Christmas Island, were getting sick, so he suspected they were getting sick because of a species of trypanosomes.
Split into simpler sentences: In the 1900s, a parasitologist had noticed that "Rattus macleari", a species of rat endemic at the Christmas Island, were getting sick. It was suspected they were getting sick because of a species of trypanosomes.