The correct answer to this open question is the following.
One way in which the understanding of the causes of disease and illness was different in 1750 from the present day is because at that time there was not the kind of scientific and medical knowledge that allows medical doctors to discover, prevent, or treat a disease.
Major discoveries such as the presence of germs and how this affected the human body, started to change the way doctors treated patients. Since those years, medical discoveries and technology in the creation of vaccines and medicines have allowed humans to live longer lives and with a better quality of life.