[SalesForce] Send Test and verify Merge field issue on HTML Email Template

There is an Idea for this: use cross-object merge fields in email templates that, as of December 2015, is under consideration. Vote it up!

Since your schema is:

Contact          Disease
   |                |
   |                |
   |                v
   ---------->  Patient

OPTION 1

You will need to send the Email on Patient and where Patient has two custom formula fields:

Patient Contact Email - defined as Contact__r.email
Patient Disease - defined as Disease__r.name

OPTION 2

Create a new custom object called Email Proxy. Update that with all the fields you need to send the email (recipient (as field of type Email), disease name, patient name). Add a field called "Is ready to send". Point the template at the email proxy object

Use workflows or process builder to send the email (because "is ready to send" is true. Reset this field to false after workflow/process executes

https://help.salesforce.com/HTViewHelpDoc?id=merge_fields_email_templates.htm&language=en_US

As you can see in tips section that

The {!Receiving_User.field_name} and {!Sending_User.field_name} merge fields work only for mass email and are unavailable in Visualforce email templates.

For single email you know the user you are sending so while sending you can reference their name directly .

If you are sending to contact use Related to as Contact and if user select Related to as User .Once you select Related To object you can define merge fields and reference same in the Email template.

{!User.FirstName}