Norovirus tends to makes the news when an outbreak occurs on cruise ships. But the virus affects many more people than ocean-going vacationers. The Centers for Disease Control and Prevention estimates some 20 million people suffer acute intestinal illness from norovirus each year in this country. It's responsible for more than half of all cases of foodborne illness in the United States It can also get into municipal water supplies, when old pipes or storm overflow cause waste water contaminated with the norovirus to mix with drinking water.