Queries :: Getting Required Data Using Query For Calculations To Be Used Later?

I've been trying unsuccessfully for weeks to try and query a usable data set that would later be used for calculated values in a report.

I'm building a Hospital Acquired Infections Database for my facility to record data and process reports automatically. Previously, I built the system in Excel using VBA and userforms, but the size of the file has become too cumbersome, and now takes too long to process my reports.

So here's the issue.

UNIT_DATA TABLE:
UNIT
INF_MONTH
INF_YEAR
PT_DAYS
CVC_DAYS
IUC_DAYS
VENT_DAYS
APV_DAYS

PT_LEVEL TABLE (There are many fields in this table for recording purposes but I will only list the ones I need for reporting purposes):

UNIT
INF_STE_MAJ
SPECIF_SITE
INF_MONTH
INF_YEAR

So the important numbers I can't seem to collect are the total number infections per month from the PT_LEVEL Table for only select SPECIF_SITE infections.

the Tables are joined Left to Right as Follows:

UNIT_DATA.UNIT - PT_LEVEL.UNIT
UNIT_DATA.INF_MONTH - PT_LEVEL.INF_MONTH
UNIT_DATA.INF_YEAR - PT_LEVEL.INF_YEAR

The UNIT_DATA Table is a monthly collection for the 5 metrics (PT_DAYS, CVC_DAYS, IUC_DAYS, VENT_DAYS, APV_DAYS) regardless of whether or not the number for each is zero. Every unit in the database will have a record for each month and year.

The PT_LEVEL Table will only have records if and when a patient develops an infection. This means there is no cumulative monthly data for the PT_LEVEL Table Data based on SPECIF_SITE infection type.

I need to first do a roll up count for every month and year in the database, for which I was initially trying to use the UNIT_DATA table for since it contains every month and year. The problem is when I try to query the SPECIF_SITE from PT_LEVEL, I can get the number of Infections for months where infections where present for each unit and null values, for each month, and each year in UNIT_DATA, but when I include a where condition to narrow the view to only selected SPECIF_SITE's it only shows data for that SPECIF_SITE for months where they occured.

I need a cummulative monthly aggregation of the totals for a selection of SPECIF_SITE infections, for every unit, for every month, and every year, since July 2010.