Infusing olive oil with herbs
See this article from Colorado University Safe Food.  The goal is to prevent growth of the botulism pathogen.  Here are their key points:


Wash all soil-contaminated produce before adding it to an oil infusion, 
Add an acidifying agent such as lemon juice or vinegar to the recipe at the rate of one tablespoon per cup of oil, 
Keep oil infusions refrigerated in order to retard the growth of any microbes, 
Discard infusions after one week, or sooner if apparent cloudiness, gas bubbles, or foul odor develop and, 
When in doubt, throw it out. 


Sorry the source article is not formatted prettily, but I was looking for something more authoritative than about.com or ehow.com.