Let's discuss!
Clostridium species can produce exotoxins.
4 things inhibit Clostridium in wort/beer:
- Low pH (<4.5 completely inhibits)
- Oxygen (even slight amounts)
- Hops
- Ethanol
Clostridium won't be present in any great quantity, since it's strictly anaerobic and only spreads through spores, mainly present in soil.
In unhopped wort:
Both yeast and lactic acid bacteria rapidly lower the pH. Oxygen is the key factor that prevents Clostridium growth
before the microbes can take the wort/beer below the pH threshold.
Now you understand Clostridium's window for growth is negligible.
Furthermore, your body is perfectly capable of metabolizing trace amounts of toxin. Exotoxin from
Clostridium botulinum is even used clinically (BOTOX®).
Also worth mentioning, Clostridium would generally produce some pretty nasty fecal aromas if it were growing, which would cause most of us to dump that particular batch.
What might be concerning?
Mold growth is problematic because it's allergenic and can produce toxins/carcinogens, however it's easy to identify by surface growth. Mold requires lots of oxygen, so once yeast fermentation begins, mold growth is no longer a possibility, assuming you use basic techniques to prevent oxidation.
There's also a theoretical possibility that enteric bacteria (coming from a fecal contamination) could survive (but not grow) in beer for a couple of days to weeks. A number of things would need to go wrong for you to get sick: fecal contamination of your beer (variable likelihood), a strain would need to be pathogenic (very unlikely), you'd need to consume it by tasting the beer before it dies (unlikely, unless you decide you want to drink the whole batch after a couple days of fermentation), and it would need to survive your stomach (maybe if you're on an acid-suppressing medication).
Lastly, there's never been a reported case of microbial-related food poisoning from beer.
Cheers
EDIT
Forgot Staph.
Staphylococcus can produce exotoxins.
Staph is very sensitive to both pH (<5.1) and ethanol (0.1% ABV or less), so it's inhibited even at normal post-boil wort pH (typically 5.0-5.2).
It's gram positive, so hops inhibit it as well.
I suggest 1.035-1.040 wort, pre-acidified to pH 4.0-4.5.
The procedure I recommend eliminates any risk to your health.
Fruit juice is also generally under low enough pH to prevent toxins.