Botulism bacteria (Clostridium botulinum), found in dirt, water and oceans, produce very dangerous toxins when it is allowed to multiply and has little contact with oxygen. These toxins are extremely dangerous, but exposure to botulism is rare in humans. Low-oxygen environments include those created in ferments, preserves or oily environments.
Although it’s rare, this bacteria can cause your delicious home-made garlic-infused olive oil to make you sick if you use it uncooked, for example in dipping or for salads.
There are a number of cases recorded of people getting sick after eating their own home-made garlic-infused oils. The symptoms include dizziness, blurry or double vision, difficulty swallowing, breathing and/or speaking, and a gradually-worsening paralysis. You don’t want botulism.
You won’t know if your garlic has botulism growing in it, because it doesn’t alter the taste or smell, which is why it is recommended to gently heat the oil to at least 85 °C (185 °F) for at least five minutes. [1. “Botulism Fact sheet N°270”. World Health Organisation. August 2013. Retrieved 5 April 2015.]
This won’t damage good-quality olive oil because the smoke point, even of poor-quality olive oil, is about 120 °C (250°F). It is recommended that you make and use oil weekly to avoid any potential issues.