headdy,
>Had this gone unnoticed and water use was continued, what diseases etc could arise from the contaminated water that is filling my sink and dishwasher?<
There are a host of deadly disease causing bacteria, viruses, and protozoans that are transmitted by contact with human sewage. Just to name a few, HIV (AIDS), Typhoid , Cholera, Cryptosporidium, and E-coli O157:H7.
John Aldrich (Septic Tank Yank)
Septic System Consultant
Timnath, Colorado