1 - I believe environmental issues have the most influence on one's health. Many diseases can be formed from environmental danger, such as; respiratory diseases, heart diseases, cancers, air pollution, etc...
(if you aren't looking for driving accidents I will put different accidents as well)
2 - There are many things you can do to prevent accidents; being sober while driving, going the designated speed limit, do not text/call while driving, looking both ways while crossing the road, wear seatbelts.
(Second Option)
Keeping an eye out for electrical and fire hazards, checking cord/electrical wire for any tears and/or rips, while using medicine be sure to read and understand the official use of the medicine, keeping flammable objects away from sources that produce a lot of heat (IE: heaters or stoves)
In the case of an accident; it would be best to stay calm, calling 911 (this will be different depending on where you live, if you do not live in the US, research the emergency service number for your country.) Making sure to tend to life-threatening wounds with a first-aid kit, if you don't have a first-aid kit, fabric from clothes would also help.
3 - Environmental Health: Water pollution, air pollution, dangerous/extreme weather condition and chemical exposure (carbon monoxide, ammonia, chlorine, hydrochloric acid, and sulfuric acid)
Consumer Health: Mis-using a product, raw meats (chicken, pork, beef, etc..) non-FDA approved food/drug products, disease infected food and water with major bacteria.
(Hope this is good enough, tell me if I misunderstood anything!)