Barry Feldman wrote:

we have one shower in the house. (I had the cesspools checked and the
cesspool guy said they're not the problem)

It doesn't smell until you start the shower then this acrid smell comes
out of the drain that's pretty awful then it goes away after about a
minute.

The cesspool company said maybe it's a venting problem. My plumber said
it couldn't be something about the trap and how it's situated vis a vis
the vent( I didn't understand)

the plumber suggested pouring bleach down the drain that there was some
stuff in there. It didn't help! The trap and the piping under the
shower is new PVC.

Remove the drain cover and look down with a bright light.
Can you see the water in the trap?
Yes? Then the trap is working and "trapping" sewer gases.

Flush the toilet while watching water in the trap.
Does the water remain relatively still? Doesn't get sucked out?
OK? Then it's not a venting issue.

Usually shower drain odors are caused by mold/mildew growth
on soap scum which is located *above* the trap inlet. Hot water
from showering releases noxious gases from the molds.
The fix in this case is a thorough scrubbing (toothbrush) with
a bleach solution to get rid of the scum. Just dumping bleach
down the drain won't get rid of it.

Jim