The ancients had no idea that close to 200 species of bacteria — both good and bad — live on human skin. But they, too, were preoccupied with keeping themselves looking — and smelling — clean. Our concern with personal hygiene (the word is derived from Hygeia, the Greek goddess of health) predates the elaborate bathing rituals of the Greeks and Romans and goes back to Neolithic times, according to British historian Virginia Smith.

The ancients had no idea that close to 200 species of bacteria — both good and bad — live on human skin. But they, too, were preoccupied with keeping themselves looking — and smelling — clean. Our concern with personal hygiene (the word is derived from Hygeia, the Greek goddess of health) predates the elaborate bathing rituals of the Greeks and Romans and goes back to Neolithic times, according to British historian Virginia Smith.

And there's more to cleanliness than appearance and odor, she writes in "Clean: A History of Personal Hygiene and Purity" (Oxford, $30). It is appealing. The process of being groomed, she says, "produces mildly narcotic effects; the longer it carries on, the more swooning or relaxing effects it achieves."

In her new history of Western cleanliness, "The Dirt on Clean: An Unsanitized History" (North Point, $24), Katherine Ashenburg takes a cross-cultural approach. "For the modern middle-class North American," she writes, " 'clean' means that you shower and apply deodorant each and every day without fail. For the aristocratic seventeenth-century Frenchman, it meant that he changed his linen shirt daily and dabbled his hands in water but never touched the rest of his body with water or soap." And for centuries, she says, the filth of Europeans appalled the more scrupulous Muslims.

Even if we take cleanliness seriously today, Ashenburg argues, it is a moving target. "Nothing," she says, "would change our bathing habits more quickly than a serious water shortage."

Jessica Snyder Sachs brings the battle against dirt firmly into the 21st century when our worries focus less on unsightly (and malodorous) dirt than on invisible, microscopic foes. She opens "Good Germs, Bad Germs: Health and Survival in a Bacterial World" (Hill and Wang, $25) with a horror story: the death of college football player Ricky Lannetti, whose mild, flulike symptoms suddenly send him into organ failure. The fit young man succumbs in a matter of days to methicillin-resistant Staphylococcus aureus, or MRSA, the superbug blamed for the deaths last month of a 17-year-old Virginia high school senior and a 12-year-old New York middle school student.

After exploring how our age-old relationship with microbes was altered by the miraculous advent and subsequent overuse of antibiotics, Sachs looks ahead to more prudent use of the drugs that have saved lives for more than half a century and to what their successors in the ongoing fight may be.