Look this up on the search engines and you will not find out very much. Look for beer diseases in brewing books and you will find silence (except to say that you must guard against them). Look for beer diseases in published literature and you will discover references to Louis Pasteur’s work of 1876 which pretty much covered the subject.
Unless you do something really gross there are two diseases you will come across, film yeast and vinegar bacteria.
Film yeast is insidious. Symptoms of film yeast in your home brew are:-
A dusty looking scum on top of your beer.
If sampled the affected beer will taste weak and vaguely unpleasant .
Film yeast is an aerobic yeast which destroys the alcohol by converting it into carbon dioxide and water. Alcohol importance of maintenance is its food. It does not immediately make the beer sour, but it quickly makes your home brew undrinkable.
There are three causes. Firstly the fermentation bucket was not cleaned properly after a previous infection. Secondly the beer was exposed too long to the air, which carries film yeast around. Thirdly the fermentation was too slow (actually this cause is connected to the previous one).
Sterilising with chlorine, iodine or sulfur based sterilizing agents will sort out the first. Rapid fermentation and prompt bottling will sort out the other two, so make sure that your yeast is good before you start; don’t kill it by adding it to wort that is too high tech dassault warm; don’t let the fermentation hang for lack of warmth. Lastly bottle your home brew as soon as its clear. You may want to use isinglass or egg whites to clear the home brew more rapidly. I have had numerous film yeast infections mostly due to late bottling.
As lager is slower fermenting you need to take an extra precaution: use a closed fermentation bin with an airlock. Top fermentation beers can be fermented in an open bin, simply covered with a cloth.
Vinegar infections are much less common. The bacterium converts alcohol (its food) to vinegar. The brew is immediately unpleasant and sour to drink. I can remember only one vinegar infection in 20 years of brewing.
Beer is protected to an extent by the hops which are a preservative, and both diseases attack weaker brews more easily than strong ones.
Incidentally, wine is subject to the same diseases: though stronger it has no protection from hops and so most wines are dosed with sulfur instead.