Typhoid fever, a disease that causes headaches, digestive upset, and a high fever, is caused by the bacterium Salmonella typhi. Typhoid can be spread from person to person by contaminated water or food or by a lack of cleanliness. Since the 19th century, the number of individuals infected with this disease has decreased.
Which statement best explains why the number of people with this disease and other bacterial diseases has decreased over the last 100 years?

a) Scientists have corrected the damaged genes that cause typhoid fever and other infectious diseases.
b) Public health officials have placed better controls on the use of the toxic substances that cause these diseases
c) Typhoid fever, like most other bacterial diseases, is often caused by a lack of proper nutrition.
d) Personal habits, such as hand washing, have greatly reduced contamination from bacteria.