Instead of this relationship:
Maintable::date = MedicalHistory::date AND
MainTable::Treatment = MedicalHistory::date AND
MainTable::Notes = MedicalHistory::Notes
Use this relationship:
MainTable::PrimaryKey = MedicalHistory::ForeignKey
You'll need to look over the fields already defined in your main table to identify the primary key field. It'll be a field with an auto-entered serial number and probably has "ID" as part of its field name.
In your relationship, you only want to match the related records by a field that will have the same value for all the records that are related to your parent record (MainTable). This field must then match to a field in the parent table that holds a value unique to that one record.
That worked. Thank you so much!