A bathroom that is not regularly used can often times have a strong sewer odor. Several factors can be the result. The simplest solution is to check the P-trap. This is the U-shaped pipe below your sink that normally holds a small amount of water.
The purpose of this accumulation of water is to prevent gases from exiting the sewer system and seeping into your home. If a bathroom is not utilized regularly, the water in the P-trap may have evaporated. A simple solution would be to run water in the sink and allow water to sit in the P-trap.
Another possibility is that the wax ring under the commode is deteriorating and in need of replacement. As the wax ring deteriorates, it allows sewer gases to seep through.