We have been away for a few weeks and just returned home. When we went to the basement to do laundry we found that there was a strong sewer smell coming from the bathroom area. Why is this and what do we do about it?

This is not an uncommon problem. Usually what happens is that when those downstairs facilities are not used for a period of time the water tends to be drawn out of the trap, which allows the odors to back-feed into that area. The key to a well-working trap is activity or usage. If you are going to be absent for a period of time, you can expect the possibility of this occurrence, unless you have a house sitter who can flush the toilet occasionally to change the water and keep the trap full. Your remedy is simply to flush occasionally.