What are the most harmful foods? Why?

1. Fugu

Fugu is the Japanese word for puffer fish and the dish prepared from it can be deadly toxic. The ovaries, intestines and liver of fugu contain tetrodotoxin, a neurotoxin up to 1,200 times more deadly than cyanide.
A lethal dose of tetrodotoxin is smaller than the head of a pin and a single fish contains enough poison to kill 30 people. If improperly prepared, fugu can paralyze motor nerves and cause fatal respiratory arrest.

Japanese chefs must undergo years of training to obtain a license to prepare fugu. Despite these precautions, many people die every year from improperly prepared fugu.

 2. The Ackee fruit

Ackee, the national fruit of Jamaica, is a delicious but dangerous treat. Unripe ackee contains a poison called hypoglycine. To be safe to eat, the fruit must be fully ripe and open naturally on the tree.
The seams on the outside of the fruit split in half when the fruit is ready to be picked.
Never open an ackee fruit yourself (it should open on its own) and eat only the cream-colored fleshy pulp around the seeds. Do not eat the pink flesh or the black seeds, as they are very poisonous.
Improper preparation of the ackee fruit can cause a serious illness (nicknamed "Jamaican Vomiting Disease"), which can result in coma or death.

3. Sannakji

Sannakji, a Korean dish, consists of live baby octopus tentacles that are cut into pieces, seasoned and served immediately.
Culinary daredevils eat the tentacles while they are still wriggling around on the plate, which is a very dangerous game.
The suction cups on the tentacles maintain suction even after the tentacles have been cut off, so diners must chew the tentacles before they stick to the roof of their mouths.
If they don't, the tentacles can stick to the mouth and throat and cause the guest to choke to death. According to Food & Wine, six people choke and die after eating (or attempting to eat) sannakji each year.

4. Hákarl

Hákarl, a traditional Icelandic dish, is Greenland shark meat that is dried and hung for three to five months. This process is necessary to neutralize the high levels of urea and trimethylamine oxide present in the shark meat.
The Greenland shark has no urinary tract, so waste and toxins are filtered through its skin and flesh.
For the shark, this mixture of compounds acts as a natural antifreeze that protects it from the frigid Arctic waters, but the chemicals are so concentrated that a few bites of fresh, untreated meat are enough to cause symptoms of extreme poisoning in humans.
Intestinal disturbances, neurological effects, convulsions and even death can occur if the meat is consumed in sufficient quantities.
Translated with www.DeepL.com/Translator (free version)

5. Cassava

Cassava, a tropical root similar to taro and yam, is often used to make pudding ("tapioca"), juice, cakes and chips, but its leaves and roots can produce deadly cyanide. 

To avoid poisoning, cassava must be properly cooked before it is canned, eaten or served.Cassava is classified into two broad categories: sweet and bitter.

Sweet cassava, which contains low levels of cyanogenic glycosides (50mg/kg), requires only cooking to reduce the cyanide content to non-toxic levels, but bitter cassava contains more toxins and must be grated, soaked and cooked properly before consumption.The Canadian Food Inspection Agency (CFIA) has provided these guidelines for safe cassava preparation.

6. Rhubarb

Rhubarb, the bright vegetable often found in jams and pies, has a dark side. Its leaves, which should not be used in baking or cooking, contain oxalic acid.

Consumption of too much oxalic acid can be fatal, although you would have to eat large quantities of rhubarb leaves (about 5 kg) to die from it.However, eating small amounts in improperly prepared foods can cause a number of unpleasant symptoms, such as a burning sensation in the mouth and throat, nausea, diarrhea, eye pain, difficulty breathing, and red urine.Oxalic acid can also cause kidney stones - which are hard deposits of minerals and acidic salts that clump together in concentrated urine - known to cause severe pain, cloudy, red or smelly urine, fever and chills.

7. Elderberries

Elderberries, which are native to Canada and commonly used in jams, pies, wines, teas, syrups and supplements, are safe to eat if they are ripe and properly cooked.

However, elderberry leaves, twigs and seeds contain potentially lethal levels of a cyanide-producing glycoside ("cyanogenic glycoside"). If elderberries are not fully ripe or properly filtered during processing, they can cause nausea, vomiting and severe diarrhea.If consumed in large quantities, glycoside can cause convulsions, coma or even death. 

You would have to drink up to five glasses to be in danger of death, but just one cup of improperly prepared elderberry products can cause serious illness.

8. Kidney beans

Kidney beans are rich in plant protein, fiber, and essential vitamins and minerals; in their raw or undercooked form, they are also rich in phytohemagglutinin, a toxic variety of lectin.Phytohemagglutinin can damage the intestinal wall and prevent the proper absorption of nutrients.
Symptoms of poisoning can include diarrhea, abdominal pain, vomiting and headaches.To be safe, dried kidney beans must be prepared properly, which involves soaking them for several hours and boiling them for at least 10 minutes.In fact, according to the U.S. Food and Drug Administration (FDA), cooking dried kidney beans for less than 10 minutes at a temperature below boiling can increase their toxicity fivefold, so they are more toxic than if eaten raw.