The Tibetan Plateau, Hindu Kush mountains, and the Himalayas comprise a formidable physical barrier to human crossings. Not to mention hundreds of miles of sparsely-populated area that would naturally slow or halt disease spread.
That makes me wonder if pre-modern plagues and diseases ever really passed between India and China? Over land, anyway?
EDIT:
The famous Black Death in the 1300s (bubonic plague) started in China and swept westwards across Eurasia to the Atlantic Ocean. North Africa, the eastern Mediterranean coast, and the Arabian Peninsula were also affected. But that particular plague never made it to India.
Another, lesser bubonic plague epidemic started in the mid-1800s in China. By the 1890s, British ships sailing out of Hong Kong spread this plague to India. Over 22 million people in British India succumbed to the plague by 1930.