I know that it seems a little confronting now but in a mere couple of months she will be rolling around on the floor and putting all sorts in her mouth, half of which you wont know its origin!
Unfortunatly we cant wrap our bubbies in sterilised cotton wool, as much as we want to...so while it does seem rude to you that the lady put her hand in your babys grasp, we cant avoid germs and viruses getting to our babys.....air borne viruses and bacteria cant be avoided, for example, pollutants in the environment....the ladys hand, whilst an unknown entity, is just one of HUNDREDS of potential sources.
I have personally been of the opinion that Ill let my kids play in the mud or dirt, as I really believes it improves their immunities (along, of course with a healthy diet and plenty of exercise)...not that I would let them swallow bucketfuls of mud or eat every insect that crawls their way, lol!....but the more that they get exposed to the more that their bodys immunity builds up a natural resistance to these things..thats just my personal opinion though...I know that every child is different!
But yes, I can understand your disdain with the situation you were confronted with, but within a couple of months, when she starts crawling around everywhere, she will be handling and swallowing stuff that you wont even be aware of!!

I found this little analogy I heard once very amusing....and quite true!
If your 1st child drops their dummy, it gets sterilised and boiled before being put back in thier mouths
2nd child will get a rinse under the tap
3rd child will get a wipe on the shirt
4th child will get a clean from the dog!!!

Best of luck to you and your bubby