How are diseases spread?

My village of about 350 was short on laborers for my new expansion, so I accepted 30 nomads in the town hall. Almost immediately, there was a cholera outbreak. I was not worried, as I'd dealt with diseases successfully before, and I had two hospitals with physicians and plenty of herbs. However the disease spread very quickly and eventually killed off my population.

I am wondering about the mechanics of diseases - logic would dictate they spread as they do in life, through proximity between villagers. Is the key to treat diseased individuals immediately and hope they did not come in to contact with anyone? Are some diseases more potent, deadly, or contagious than others, or is the difference only in the name and they are mechanically identical? Are villagers able to be contagious before they are marked as diseased or are they marked immediately upon contact? Do disease events only come up when nomads arrive?

Solution 1:

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.

Solution 2:

I believe the wiki now has all the information I wanted to know.

Regarding proximity: The diseases will spread from citizen to citizen based on health factors and proximity.

Differences between diseases: The population can suffer from diseases such as influenza, plague and everything in-between. Some diseases spread quickly, some linger, some have high fatality rates, some low.

Disease event occurrence: The population may get sick naturally, or as the town grows as a result of traders or nomads coming to the town.

Full text from the wiki:

The population may get sick naturally, or as the town grows as a result of traders or nomads coming to the town. The population can suffer from diseases such as influenza, plague and everything in-between. Some diseases spread quickly, some linger, some have high fatality rates, some low. The diseases will spread from citizen to citizen based on health factors and proximity. Physicians and Herbalists can help keep the population healthy and cure the diseases in some cases.

Taking in nomads significantly raises the risk of an outbreak of a disease.

http://banished-wiki.com/wiki/Disease