If you can make the case that soap is an engineering innovation that is used to prevent diarrhea, the rules would not stop you from submitting an essay about it. However, soap has been around much longer than the formalized engineering profession, so you may be better off looking for a certain kind of soap, perhaps one specially developed for fighting disease in the modern age.
The rules ask you to describe how engineers were involved in creating an advance in the treatment or prevention of your selected affliction. You should describe why this engineering advancement is particularly important, how individuals from different engineering disciplines have contributed to the technology, and why that matters.