It’s a complicated answer, but a water conditioner can be anything that treats your water, from a basic point-of-use filter to a water softener to a system that electronically treats the water so it does less damage to your plumbing system. Which system is best for you is a matter of what your goals are and what you want to spend. Our technicians will be more than happy to discuss your needs and give you some options for the best system for you.
The most likely reason would be that the trap is dry. This is a common problem in guest baths due to infrequent use. The water in the trap can slowly evaporate without regular use of the shower, thereby permitting sewer gas to rise into the house. The simple solution in that situation is to run your shower every few weeks to keep some water in the trap.