Cumulative distinct count

Hello, 

I need to create a table (using SUMMARIZE) and I need one of the fields to calculate the cumulative distinct count of people that have been tested. My data is similar to this:

This is what I expect to have as the table outcome. So for the month #3, I want to count all the people that have been tested so far in month 1, 2, and 3.

All I have been reading is to tacke this via a measure, but I need to create a table for this purpose. 

Thank you for your help!