Complex sentence: Harris was forced back into the workforce because of a disease, believed to be malaria, he contracted on the Walcheren expedition in 1809 that forced him out of the military and unfit to serve, costing Harris his pension.
Split into simpler sentences: Harris was forced back into the workforce because of a disease, believed to be malaria. He contracted on the Walcheren expedition in 1809 that forced him out of the military as unfit to serve, costing Harris his pension.