Non-symptomatic cases make newly identified diseases less scary
If you've been following news about the H7N9 bird flu outbreak in China, it may be relieving to know that doctors are now looking for (and finding) people who are infected with the virus, but who appear perfectly healthy
or who are just suffering from a mild case of the yucks. It's an important reminder that we identify new diseases when sick people show up, very sick, in hospitals. Just because those are the only people we know to have the disease, doesn't mean the disease makes EVERYONE that sick. Hidden in the background are often many, many people who shrug off a new flu the same way you or I have shrugged off an old, boring flu. This is context you should take into reading about every new disease.