Complex sentence: Nicholas was thought to have a strong constitution until early 1865 while on tour in southern Europe he contracted an ailment that was initially incorrectly diagnosed as rheumatism which later turned out to be cerebro-spinal meningitis.
Split into simpler sentences: Nicholas was thought to have a strong constitution until early 1865, when during a tour in southern Europe he contracted an ailment that was initially incorrectly diagnosed as rheumatism. It later turned out to be cerebro-spinal meningitis.