Just scrape the mold off, right? Wrong. A moldy old sandwich like this one is anything but harmless. While there are some harmless kinds of mold - like on Camembert cheese - many molds are toxic. Furthermore, mycelium spores can trigger allergies. Through contact with highly toxic types of mold, humans with weakened immune defenses could even die as a result of an extended exposure.

Mold can also be useful: Fungi is able to break down carbon hydrates, fats and proteins - more efficiently than any other organism. Industry makes use of a genetically modified Aspergillus niger fungus, which produces enzymes that can be used in food processing and production of detergents - like a living factory.

"Botulus" is Latin for "sausage." If mistakes are made in the production of sausage, or if meat or vegetables get contaminated during canning, this can cause botulism. The bacteria Clostridium botulinum causes this life-threatening poisoning.

Clostridium botulinum thrives in anaerobic, or oxygen-free, environments. It produces the nerve agent botox - used among cosmetic surgeons to help smooth skin. But in food, it leads to paralysis. At first, certain body parts get paralyzed, resulting in symptoms like slurred speech. But later, also the muscles responsible for breathing and heartbeat freeze up - eventually resulting in death.

Fenugreek sprouts were a favorite among Germans trying to eat healthy - until 2011. That year, seeds contaminated with the bacteria Escherichia coli (EHEC) caused an outbreak that killed 53 people - hundreds more were sickened. EHEC produces a toxin that destroys intestinal wall cells, and later attacks brain and kidney cells. Cooking raw vegetables and meat kills the harmful bacteria.

But not all varieties of E. coli are dangerous. Inside the human large intestine, the bacteria are usually responsible for producing vitamin K - important for the development of bones and cells, and for blood coagulation. In biotechnology, the bacteria play a role in producing insulin and growth hormones. They can even be used for turning microalgae into alcohol-based biofuel.

Thousands of years ago, humans learned to use lactic acid bacteria - for the production of yoghurt, kefir, sourdough bread and cheese. Raw milk warmed to 20 degrees Celsius is heaven for bacteria: Within 10 hours, the milk will go sour. Milk fermented with the help of bacteria, however, can stay edible for much longer.

One of the many varieties of lactic acid bacteria are streptococci, which play a role in producing sauerkraut and fermented milk products. Although streptococci are everywhere - on humans, animals and plants - some of them are unhealthy. Some strains of strep can trigger tooth decay or sepsis, commonly known as blood poisoning.

Rod-shaped bacteria like Campylobacter and Salmonellae cause illness and death the world over. Undercooked beef, pork or chicken containing Campylobacter is a common cause of diarrhea wordwide. Typhus is the most dangerous form of salmonellae, triggering high fever, weak heartbeat and constipation. Every year, about 32 million people are sickened from typhus - mainly by drinking impure water.

Norovirus or stomach flu is transmitted person-to-person through traces of vomit or feces. Just 100 tiny norovirus particles are enough to infect someone. The virus can easily pass into the food chain via infected drinking water.