Rotten smell whenever a shower is taken.

Whenever someone takes a shower on the second floor of my parents house, there's is a rotten smell that comes from the septic tank. I was wondering what this could possibly mean and what could be done about it.

Need more detail. Where is the odor coming from? The shower drain?

Could be lots of things. Sure it's the septic tank and not a vent from the house near the tank? Could be field lines stopped up and causing fumes to come thru the earth over the tank more than normal. If there is not a good seal on the septic tank then fumes could be coming from between the lids on the older type tanks. In the past alot of installers covered the tanks with roofing felt (tar paper). Lots of places now use one solid lid that has openings for servicing and lids seal around the edges for practically an air tight fit. Surely others will weigh in on this topic.