Nearly 500 dolphins have washed up on the eastern seaboard this year, almost four times the usual number, and the mystery has left many oceanographers scrambling for possible causes. But after intensive testing, the National Oceanic and Atmospheric Administration has found a likely culprit: morbilivirus. It's a measles-like contagion that can cause pneumonia-like symptoms in dolphins, and has been linked to previous die-offs in 1988 and 1992. NOAA is reporting that all of the 27 carcasses they've examined have either tested positive for the disease or are suspected for infection based on visible symptoms.

Just weeks ago, NOAA officials were stumped, telling CBS News, "it could be biotoxins. It could be disease. It could be human interactions with fishing gear." Toxins and human intervention haven't been ruled out, but given the positive tests and the disease's history in similar mortality events, an aquatic measles epidemic now seems to be the most likely explanation.