Hi,
As an alternative to a "mask" in Varanasi (or anywhere where there is pollution) can I recommend "Buff" scarves which can be used for all sorts of occasions. I always wear one on the plane - some airlines spray the plane and also you might have someone next to you coughing and sneezing.
I am about to go to India for the 5th time (not with JY), but I did go on the Spirit of India which is a fantastic trip. As well as Imodium, you will need anti-bacterial gel - you need to use it frequently, especially after handling money, and also wet wipes.
Sorry just seen that you are going today, but hopefully anyone going on this trip for the future will pick this up!
If you go up to the Search button (top RH of this page) and put in "India" and Reviews, you will see a wealth of advice.
You will either love India or hate it - it is not called "Incredible India" for nothing! When I went some years ago on this trip, they did not have the Add-On to Amritsar, so that is where I am going with another company.
Enjoy!
Cheers,
Hils