Don't use insecticides on garden vegetables immediately before harvesting, and wash the vegetables when you pick them. Try not to damage your plants if they are still producing. (For root vegetables, this is unavoidable.)
Make sure the surface where you prepare them is not contaminated by liquids, meat juices, or soaps -- these can transfer a bad taste to the food. If you want to maximize the nutritional value, don't peel them before cooking, and don't overcook. Raw vegetables have the most nutrients, but obviously many do need to be cooked, or at least steamed.
Freezing can save vegetables for future use, but can destroy the texture and make them mushy. There are several methods of preserving (canning) fresh produce.
Personal hygiene is imperative when cooking. This helps prevent foodborne illness and similar factors.
Most people simply use the frying method while cooking their vegetables.
Steaming the vegetables rather than boiling.
you rinse vegetables or fruit before cooking
Many people in the world prefer to steam vegetables while cooking them.
I'm sure there are details of foodborne and waterborne illness that will be specific to Kenya and this sounds like the subject of a research paper. But foodborne illness anywhere can be improved by preventing contamination (which involves the many steps from farm to fork), proper storage to prevent pathogen growth, and proper preparation (including cooking) to remove pathogens that might have hitched a ride. One of the big factors in controlling foodborne illness in Kenya is improving water quality. Even programs encouraging the washing of hands have made improvements. See Related Links.
vegetables
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Carryover cooking can affect anything. For vegetables, carryover heat can take perfectly cooked and pristine vegetables and make them bland and overcooked.
Fibre.
Foodborne infections are caused by micro organisms in the food itself. They can be viruses such as hepatitus A, bacteria, such as salmonella or parasites such as trichina. Most foodborne illnesses can be prevented by proper hand washing methods, thoroughly cooking the food and cross contaminiation prevention.
To achieve even cooking in vegetables or meats, cutting them approximately the same size will make cooking easier.