Mother nature is one twisted old crone. Sure, she's all cute baby animals and pretty flowers and impressively intricate termite colonies, but mark my words, that lady's also got a toxic side. From tiny microbes to towering trees, there exists in this world a multitude of organisms working 'round the clock to synthesize substances capable of killing other living things, including us.
Take bacteria. Within a single genus, Clostridium, you've got:
Clostridium botulinum
Maker of one of the most toxic substances known to exist, botulinum toxin. This badass protein binds to and disrupts the action of nerves that control muscle movement, resulting in paralysis. Death occurs due to asphyxiation, since your diaphragm and all the other muscles that you need to inhale stop working.
Clostridium tetani
Responsible for tetanus (i.e. rusty nail disease), due to its production of tetanospasmin. This downright sinister protein binds to neurons in your brain and spinal cord, preventing the release of inhibitory chemicals that normally keep them from becoming overactive. Hence, your central nervous system goes loco, leading to seizures and tetanic muscle spasms (your muscles contract for long periods of time, during which you are paralyzed) that are so friggin' strong that they can snap your bones. The muscle paralysis occurs first in your face and jaw, producing a characteristic face paralysis (risus sardonicus) and lockjaw.
Clostridium difficile
The scourge of hospitals everywhere, this mofo produces two toxins called enterotoxin and cytotoxin that destroy the cells lining your intestine, resulting in diarrhea and inflammation. Responsible for a very considerable number of hospital deaths and resistant to lots of antibiotics.
Clostridium perfringens
Produces toxins that can cause exciting things like food poisoning and severe infections featuring gas gangrene.
I'm telling you, bacteria are a venerable gold mine of toxic compounds. Some other bacteria and their toxins that are worth noting:
Vibrio cholerae
Causer of cholera, which features intense bouts of diarrhea. So intense, in fact, that if left untreated, you can die within hours of the first squirt since you become so severely dehydrated. The trots are the work of cholera toxin, an enterotoxin that acts on the lining of the small intestine.
Bacillus anthracis
Produces three distinct proteins that together are referred to as anthrax toxin. None of the three cause problems on their own, but when they are brought together they become a neigh-unstoppable force of utter pestilence. Anthrax is interesting because it comes in three different flavours, depending on how the bacteria gets into you. You breathe it in, you get pulmonary anthrax. You eat an infected cow, you get GI anthrax. You smear some bacterial spores in a cut, you get cutaneous anthrax. All three forms can kill you dead.
Corynebacterium diphtheriae
Diptheria comes in respiratory and cutaneous forms. The cutaneous form is pretty gross. Google it if you dare. Diptheria toxin gets into cells and inhibits their ability to make proteins, which is a pretty essential part of being a healthy and normal cell, resulting in cell death. As this is a generally bad thing, you get sick.
Streptococcus pyogenes
Not content to just produce a single toxin, this bacterium produces a whole wack of 'em. The most exciting of these are streptolysin O and S, which destroy blood cells. Toxins from this bacterium are responsible for a number of diseases, including strep throat, impetigo, scarlet fever, necrotizing fasciitis, and toxic shock syndrome.
