There is always a lot of talk about whether or not it is safe to use raw garlic in sous vide. I figured I would address the concerns and go into a little more detail about the issue.
There are two key points when using raw garlic: flavor and safety.
The first discussion point with using raw garlic, and the quickest one to discuss, is whether or not raw garlic in sous vide actually tastes good. Most of us are used to adding raw garlic when we cook meat so it is a natural transition to start including it with sous vide.
However, there is a major difference between cooking meat sous vide and roasting or pan frying it, and that is the low temperatures used in sous vide. Because sous vide meat is cooked using low heat, any garlic in the bag won't actually "cook" during the sous vide process. This obviously affects the final dish because raw and cooked garlic have very different flavors. This is also true of any aromatics like onions, shallots, or carrots.
So if you are including raw garlic in your sous vide bag you should be adding it like you would normally use raw garlic to finish a dish. It'll have a sharp, pungent flavor and not the usual cooked garlic flavor. This may be what you are looking for, and if so feel free to add it, but just be aware it will taste different than garlic that is part of a dish cooked with more traditional methods.
For this reason, many people reach for garlic powder for their sous vide cooking instead of raw garlic. You can also cook the garlic ahead of time and then add it to the bag if you want a more traditional garlic flavor, though many people just introduce the garlic after the sous vide process as part of a pan sauce or salsa. If I want a nice garlic flavor I'll often roast a head of garlic while I'm sous viding and then use that as part of the final dish.
The big question people ask is whether or not raw garlic is dangerous when used in sous vide cooking. This is because garlic can have botulism bacteria on it which thrive in low oxygen environments, such as a sous vide bag. This is typically a concern when you are canning or making infused oils, but it can also be an issue in sous vide cooking because most of the oxygen is removed from the bag.
That said, botulism only makes spores (the toxic part you need to be concerned about) in a certain temperature range that generally matches the normal danger zone in cooking. If you are following good food safety practices your food shouldn't be in this range for long enough to have any risk of botulism.
Because botulism stops making spores around 122°F to 126°F (50°C to 52°C), any long term cook you do will most likely be above that and completely safe. If you want to err on the side of safety, you can omit raw garlic when you are cooking at lower temperatures, such as for fish or rare beef.
If you are going to store your food after sous viding it, it makes sense to chill it quickly in an ice bath and then refrigerate it to minimize the time it spends in the danger zone. But again, this is proper food handling whether or not garlic is involved.
If you are leaving food out at room temperature for hours, days, or weeks (like with canning or infused oils) then botulism can become a serious issue. If you leave your bagged steak on the counter for a week then botulism isn't the only thing you should be worried about.
So the short answer is yes, it can be dangerous but it is very, very unlikely. To paraphrase one food scientist, you are more likely to choke on your food than get sick from raw garlic in sous vide.
Just follow good food safety guidelines, minimize the time your food is in the danger zone, and you should be completely safe. As I mentioned above though, the garlic will not be "cooked" so it might not impart the flavor you are looking for.
If you are interested in more information about botulism, here are some links: