Complex sentence: Less than two months after their arrival, all 57 are believed to have died during the second cholera pandemic, although it's also thought they were murdered to stop the spread of the disease which was a worldwide contagion spanning several continents and many years.
Split into simpler sentences: Less than two months after their arrival, all 57 are believed to have died during the second cholera pandemic. While most died of the disease, forensic evidence suggests that some were murdered, perhaps to stop the spread of the disease which was a worldwide contagion spanning several continents and many years.