The virus lives in the gut and is passed from person to person by poor hygiene after going to the toilet.

BBC: Norovirus caused more than 1,000 Scottish ward closures

The illness is usually caused by a virus and passed from person to person, said Dr. Corey Siegel, a gastroenterologist and professor at Dartmouth College's Geisel School of Medicine.

CNN: Queen Elizabeth II released from hospital

Gartner's Weiner says video can be passed from one person to another legally with DRM technologies.

FORBES: Magazine Article

On a more reassuring note, investigators have found no evidence that the virus has passed directly from person to person.

CNN: New bird flu well-adapted to infect people

The H7N9 strain was not previously known to infect humans before cases turned up in China this winter, and Zhong and other medical experts said no evidence exists that the virus can be passed from one person to another.

NPR: A New Case In China Adds Unknowns To Bird Flu

Mr. QUAMMEN: Contagious could be any situation in which the cancer cells from one person are passed to another.

NPR: Startling New Evidence Cancer Is Contagious

The addition of a tiny amount of DNA from a third person will be passed down the germ line - through the eggs - to subsequent generations.

BBC: Ethics of using three people's DNA to create one baby

Known as "autosomal" analysis, this test involves scanning a million points across each person's genome to look for matches in the patterns passed down to them incrementally from previous generations.

WSJ: Henry Louis Gates Jr. on 'Finding Your Roots'