Sorry about yesterday’s email’s subject line — I messed up, and I didn’t notice what I did until it hit my inbox. — Dan

The dairy industry is an enormous one. Per some estimates, the world produces more than 700 million tons of cow’s milk a year, giving us the basis for butter, cheese, and a lot more. To produce that much milk, we need a lot of cows, and we generally want those cows to be happy. Agricultural scientists, for decades, have worked on treatments to do just that. And in 1973, the pharmaceutical industry developed a drug called diclofenac, a pain killer which, as it turns out, not only helps people but also works on cows.

Unfortunately, it doesn’t agree with vultures. And that caused a massive, massive problem.

In 1994, the patent on diclofenac ran out and it became widely available for cheap. Ranchers in India were particularly eager to use the new drug — India is home to more than 500 million head of livestock, the most in the world. And as most Indians don't eat beef, the cost/benefit of keeping a herd is more difficult to balance than it is elsewhere. Farmers started to use diclofenac to treat inflammation and pain in their livestock.

Unfortunately, that “no eating beef” rule created a weird problem. Cows will, ultimately, die, and it’s not so easy to get rid of a 1,000-pound carcass. In most cases, the solution as to let the carcass just lay where it was and hope nature would take over — which, usually, it did. India had a lot of vultures at the time, and — as scavengers — they were perfectly positioned to help. A group of vultures can strip a cow carcass down to bone in about 45 minutes, leaving nothing behind to fester, rot, or spread disease. By the early 1990s, India was home to roughly 50 million of these birds, nature's sanitation crew working for free.

But then, the birds started dying. It turned out that diclofenac has a side effect no one anticipated: even trace amounts are fatal to vultures. When the birds fed on the carcasses of treated cattle, the drug caused kidney failure. Uric acid built up in their blood, reaching up to ten times normal levels, and they died. Within a decade, as CBS News reported, India's vulture population plummeted from 50 million to just 20,000 — a collapse faster than any other bird species on record, and the largest since the extinction of the passenger pigeon.

The consequences rippled outward in ways no one foresaw. Without vultures to dispose of carcasses, farmers dumped dead livestock in rivers and lakes, contaminating water supplies with bacteria and pathogens. Feral dog and rat populations exploded, filling the ecological niche the vultures had vacated — but unlike vultures, dogs and rats carry rabies and other diseases. India became one of the largest center of rabies in the world.

And that had impact on the human population, according to Anant Sudarshan, an economist at the University of Warwick who co-authored a study on the crisis. Sudarshan and his co-author Eyal Frank, an environmental economist at the University of Chicago, compared death rates in districts that once had thriving vulture populations to those with historically few vultures. The results were staggering: from 2000 to 2005, human mortality increased by more than four percent in vulture-affected districts — roughly 100,000 additional deaths per year. Over the five-year period, this accounted for approximately half a million more deaths than we would have suffered otherwise.

India banned veterinary use of diclofenac in 2006, but as the BBC noted, enforcement has been weak, and other vulture-toxic drugs remain in use. The vulture population has not recovered; three species have suffered long-term losses of 91 to 98 percent. (Vulture populations typically rebound slowly, as the birds take three to five years to mature, and typically only lay two eggs per year.) Even with aggressive conservation, experts say it would take at least a decade for the birds to bounce back.

The alternative, Sudarshan noted, would be to build a nationwide network of incinerators to dispose of the millions of animal carcasses India produces each year. That would cost about $1 billion annually, consume vast amounts of energy, and add to India's already severe air pollution problem. "So it makes more sense," he told CBS News, "to bring back the natural way." The vultures, it turns out, were doing a job worth billions — and no one noticed until they were gone.

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Today’s Bonus fact: In rare circumstances, even vegans will eat meat — well, if they’re bunny-like, at least. Snowshoe hares live in the Arctic and, typically, are avowed herbivores. But when winter hits, vegetation can be very hard to come by — and survival wins out. As National Geographic reports, “during summer months, the mammals feed on vegetation, but when snow blankets the landscape and temperatures plunge to 30 below, hungry hares scavenge other hare carcasses, as well as several species of birds.”

From the Archives: The Rules of the Roadkill, Smart Phone-Version: If you see something, say something — and then you can eat it.

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And thanks! — Dan