Do different diseases have different contagiousness / mortality rates

I believe it's better to have many small stockpiles. But you shouldn't put it near labourers. It should be placed near where the recurring production is- trading posts, quarries, foresters/woodcutters. You want these people to store their resource output as quickly as possible so they will go back to working and producing. Some stuff like firewood, you do not want your villagers walking out into a blizzard because they're freezing and need firewood but the stockpile is far away. I apply the same principle to storage barns.

The real problem is for resources that you can't reliably produce locally like iron or stone. For these resources they will have to walk a long way to get it from whatever your current source is, and there's no stock pile placement to alleviate this. But you can place your stockpiles to increase production. It's a lot easier to build more blacksmiths than to summon more merchants to get more iron.

Nomads and Traders do increase your chances of spreading Diseases to your population.

So far I've only encountered Small Pox and it did not seem spread via proximity. I was trying to watch it closely and it looked to me that it started on one side of my settled area and was isolated there, but suddenly someone on the other side of my little island was infected as well.

Best bet is simply to have a Hospital available in a semi-centralized location for infected villagers to go to. I believe it can hold 20 patients at a time.

During my Small Pox break out I was fortunate that I already had a Hospital up near by and the infected would immediately go check in. With ~175 people the infection never spread to more then 5 people at a time.