The Hygiene Motive is highly important to Sims. Bad hygiene won't kill a Sim, but it will have a detrimental affect upon their mood and their Social skills. A Sim who doesn't wash will also not get job promotions. If a sim's hygiene level seriously drops, a green mist will follow them around and other sims may hold their noses near them. A sim's hygiene level will also drop if they wet themselves, or if a baby throws up on them.
 How To Resolve
A Sim can wash their hands, take a bath or have a shower to raise their Hygiene Motive. It's worth noting taking a bath can also raise their Comfort Motive. It is also possible for a sim to sponge themselves if they are at a sink.