Symptoms

The symptoms of food poisoning usually begin one to three days after eating contaminated food. They include:

Feeling unwell
Vomiting
Diarrhea
Stomach cramps
Other symptoms of food poisoning include:

Abdominal pain
Loss of appetite
A high temperature of 38C (100.4F) or above
Muscle pain
Chills

Causes

Food borne illness usually arises from improper handling, preparation or food storage. If micro-organisms are transferred from one surface to another especially in case of ready-to-eat foods or raw foods.

Cross-contamination: Cross-contamination is a cause of food poisoning that is often over cooked. It occurs when harmful bacteria are spread between food, surfaces and equipment. Most common organisms that cause food poisoning:

Campylobacter jejuni
Clostridium perfringens
Salmonella spp. – S. typhimurium infection
Escherichia coli (E. coli )
Virus (noro virus)
Parasites (Toxoplasmosis)
Mushrooms and toadstools: Dozens of species can cause muscarine poisoning. These poisons attack the central nervous system, causing partial or complete paralysis in severe cases.
Fish: Some fish, like the puffer fish, are naturally poisonous. A poison similar to that naturally found in the puffer fish can also occur in many edible Caribbean and Pacific species. It's called ciguatera poison, and it's produced by a tiny sea parasite called adinoflagellate. This poison attacks the nervous system.
Insecticides: There are many types of poisons found in insecticides but the most dangerous types are the organophosphates, which are basically nerve gas for insects. Such insecticides are deliberately formulated to be less harmful to humans than insects, but these chemicals can be very dangerous to people if the insecticides are not used properly.
There are many other causes of food poisoning. These include wild nuts, leaves, flowers and berries, under ripe tubers, botulism, cadmium from containers, lead or arsenic from fertilizers, and acids and lead from pottery.

Diagnosis

Generally symptoms are used to diagnose for the food poisoning. These may include pain in the stomach and signs body does not have as much water and fluids as it should (dehydration).

Stool test: Sometimes stool test is done for bacteria or parasites.

Treatments

In most cases of food poisoning, symptoms can be relieved at home without seeing physician.

Oral rehydration salts: If diarrhea is persists, oral rehydration solution such as Oralyte, should be drunk to replace the fluid losses and prevent dehydration.

Drink plenty of fluids: This is to prevent dehydration

If condition persists visit your doctor.

Preventions

Some do’s and don’ts to prevent food poisoning at home:

To wash hands with warm soapy water for at least 15 seconds before eating or preparing food.
Utensils and cutting board should be washed with warm soapy water and disinfectants.
Keep raw foods (raw meat, poultry, shell fish) and ready to eat foods away from other foods to avoid cross-contamination.
Do not undercook foods. Reheat food rapidly until all parts of the food reach 75 degree C. Cook fish and meats at 145 to 165 degree F.
Food should be discarded if there is uncertainty in its quality or safety.
If you are suffering from food poisoning:

Avoid dairy products that might worsen diarrhea.
Avoid eating solid foods.
Avoid alcohol, smoking, and fatty foods.
Replace fluids lost by diarrhea or vomiting by drinking water or other fluids except milk and caffeinated beverages.
Don’t take antibiotics or anti-diarrhea medications before consulting a doctor. Certain types of food poisoning (viz. listeriosis) are treated with intravenous antibiotics.
Consult doctor if diarrhea doesn’t get better in 5 days (3 days in case of children)

References:
FoodSafety.Gov
NHS
WHO