When you don't understand illness or disease, demons and spirits seem to be a good answer. I was watching a series called Vikings a while ago which you've just reminded me of. The Vikings went to England and found a monastery, which they raided and killed all the monks except for one, who they brought back as a slave. In one episode a character is explaining how Odin, the All Father, created the world. The monk, being a christian, protested. The Viking looked at him and said "Well how do you think the world came to be?" and the monk didn't say anything. It made me chuckle when I watched it, arguing over which god it was. But there's a point to it. When you don't know how something happened, it's very easy to accept that which makes sense to you.
Yes, there are many reasons why we may be inclined to the notion of some sort of God, I only touched on a couple. This is the 'God of the Gaps' viewpoint, which I see being increasingly attributed to questions we may never be able to definitively answer such as how consciousness arose.
Running with your comment, I'm not sure we are predisposed. Those who don't teach their children religion don't seem to have this, which in the UK and Europe as a whole is easy to see considering we aren't the most religious bunch. What I see over and over with my non religious/atheist friends and family once they have children is that their children have no concept of god until they have someone teach it to them, usually in school. I would be quite curious to find out what the trend would be if there were a group of children who were raised and throughout their lives never had the concept
However, predispositions are not always realised. It simply means we, as human beings, are susceptible to the idea of some supernatural force in the world.
Which reminds me, something else that lends credit to us not being predisposed to supernatural belief. The Pirahã tribe in the amazon have no concept of god. There was a christian missionary named Daniel Everett, went there to try and spread the gospel and after 20 years of trying to convert them ended up becoming an atheist.
This doesn't necessarily mean they had absolutely no concept of the supernatural, or superstition which arises from it. When we use the term God, it is in a very broad sense, not necessarily the classical definition.