NEW DELHI — A few days ago, Prime Minister Narendra Modi cleaned a street holding a long-handled broom, which is too benign an implement to clean India with. But Mr. Modi was only setting an example, and he had to maintain a dignified posture. He had just started what appeared to be India’s most serious attempt yet to clean itself.

His government has indicated that it will spend billions of rupees over the next five years to clean up thousands of towns and villages and keep them clean. And to accomplish Mr. Modi’s campaign promise that within a few years every Indian will have access to a toilet. He is “pro-toilet,” as the British satirist John Oliver said earlier this year.

The rich are now interested in their nation more than ever, and the poor know that they would be the first to die in an epidemic. So Mr. Modi’s drive has won immediate support across all layers of the society.

Most of India is clean indoors, but in plain sight it is one of the filthiest nations on earth. Indians have been debating inside television studios and outside why that is so. Why is it that a nation of clean homes has dirty public spaces? There are many substantial theories. But a more useful question might be: Why is the Delhi Metro rail system so clean?