Hygiene is one of the five Needs that your character must maintain. After a while, Urbs start to get dirty, and if they get very dirty, they start sniffing themselves and saying, "Ewww!" Dumpster diving will cause your Hygene meter to deplete significantly. Bad hygiene won’t kill a Sim, but it will seriously affect their mood and their social skills – bad hygiene won’t get a Sim promoted at work either. Bad hygiene can occassionally lead to your character dying by flies. Therefore, it is necessary to take a bath or shower when the Hygiene meter slips into the red – a hot tub also works, but not as well, although it can boost the Social and Fun bars. Washing one's hands after going to the bathroom is another way to boost the Hygene meter, but only by a small fraction.
Items That Boost Hygene
See the Plumbing page.