Every brew is infected. It just depends on what and how much.
Many infections will multiply at 10 times the rate of your yeast. The by-products from the infections are often detectable at very low levels.
You need to get your yeast in there working before anything else gets a chance to build up. Pitch a good healthy yeast in the right quantity.
Your yeast will protect your beer by removing oxygen, producing alcohol and dropping pH levels, which many infections do not like.
Cleanliness and sanitation are critical to brewing.
Usually brewers do not sterilise equipment, this needs an autoclave to achieve, then as soon as you remove the item from the autoclave, it gets dust on it which is a source of spores and infections. Yeast farmers do sterilise equipment in pressure cookers and use aseptic methods to transfer small amounts of yeast.
Wash every surface thoroughly with a decent cleaner. Rinse thoroughly. Then use a good no rinse sanitiser. Store everything clean and dry. Keep the outside of your fermenters clean, if you dribble some wort on the outside, this is a great spot for an infection to build up, then it may be transferred to your brew when racking or bottling.
Change your cleaner and no rinse sanitiser around to keep your local bugs on their toes. A spray bottle full of sanitiser is easy to use on everything. On brewday, I use a bucket with 4 litres of no rinse sanitiser made up. This is used to rinse fermentrs and somewhere to throw in airlocks, grommets, spoons etc. That way the spoon never lays on the bench.
Your finished beer can resist some infections, but not everything. People report finding weird things in their fermenters and bottles and the brew is not infected, then other brewers go to great lengths to find the source of an infection.
Tipping a batch out is a sad day, but most brewers have been there and done that. Just ask Tony, I think he threw out 400 litres.
If all your brews have a similar sour flavour and aroma, they are most likely infected.