Now she wondered whether one of those companies had erroneously profiled her as an M.S. patient and shared that profile with drug-company marketers. She worried about the potential ramifications: Could she, for instance, someday be denied life insurance on the basis of that profile? She wanted to track down the source of the data, correct her profile and, if possible, prevent further dissemination of the information. But she didn’t know which company had collected and shared the data in the first place, so she didn’t know how to have her entry removed from the original marketing list.

In our data-happy society, the case of the mistaken M.S. patient illustrates a lack of visibility for people interested in how information about them changes hands. When consumers fill out warranty cards, enter sweepstakes, answer online surveys, agree to online privacy policies or sign up to receive e-mails from brands, they often don’t realize that certain details — linked to them by name or by customer ID code — may be passed along to other companies. That can make it hard for people seeking to correct errors to find the keepers of their marketing profiles.

I was ruminating on that problem in Washington on Wednesday, when I paid a visit to Senator John D. Rockefeller IV, the West Virginia Democrat, who has made consumer privacy one of his signature issues.

Americans, he says, should be allowed “to be left alone.”

In part, this belief stems from his own nature. He describes himself as an introvert, a person who would rather stay at home and listen to Bach fugues than attend Beltway shindigs. But mainly, he just thinks that privacy is a fundamental American right.

“The principles are remarkably simple,” Mr. Rockefeller said, comfortably installed in an armchair in his Senate office. Americans, he continued, should have the right to decide what kind of marketing material is sent to them and what other people know about them: “People have the right to be private insofar as it’s possible in the modern world.”