I've been reading about infused oil and botulism (e.g. here: http://everything2.com/index.pl?node_...), and it seems that a general guideline to avoid botulism is to refrigerate the oil, and use it within one week.
Now, I just bought a pack of butter, and apart from salt, it contains no preservatives. Still, if refrigerated, it claims it will keep for about three months. So apparently, no risk of botulism there.
So I suppose it's the addition of the herbs that's the problem in infused oils. They are likely to contain spores, I guess, and also contain water which the bacteria need. However, butter also contains water, so that leaves just the spores as the only difference between butter and infused oil.
But then, since these spores are so hard to kill, I guess it's not very unlikely that some spores enter my pack of butter some way or another. For example, I might use a knife to cut some salad, and then use that same knife to take some butter. The risk is probably not high, but at least in some cases, this should transfer a few spores to the butter. And since the butter is comparable to infused oils in terms of botulism growing conditions, this might turn my butter into a deadly poison within a few weeks.
But I've never heard or read any warnings about botulism in plain butter. So my question is: Why is this not a problem? Are there other differences between butter and infused oils that I have overlooked?