The main idea is what the early settlers did to help us
European traders were attracted towards India because of the spices grown in tropical climates - pepper, cinnamon, nutmeg, dried ginger etc. These spices had become an important part of European cooking. Also, they found the cotton cloth very attractive.
Old Major is the allegorical figure for Karl Marx or Vladimir Lenin, depending on your interpretation of the text. Both men stood for basically the same thing, and these things was what Orwell was exploring in the novel.
Answer: Because they had never developed immunities to the disease
