As many as 3.7 million Americans are classified as preppers or survivalists. Being a “doomsday prepper” is not easy — or cheap. These people prepare for different disasters for different reasons. Most think that natural, financial or other disasters will destroy our infrastructure or tear society apart.

Whether they prepare for long or short-term emergencies, at the very least survivalists plan to have the basic necessities: food, clothing and shelter. Many prepare for much more than the basics with self-defense and security measures, power systems, water filtration, communications and bunkers. Given their numbers and their willingness to spend money, catering to doomsday preppers has become a multibillion dollar business. Regardless of which item is being considered, each $1,000 of expenses for just 1 million people comes to $1 billion.

Click here to see who is making money on doomsday preppers

To identify the industries that benefit the most from doomsday preppers, 24/7 Wall St. focused on key spending categories. Many items are actually impossible to flag as “prepper supplies” because they are ordinary household items bought by consumers every day. Other items are more specific and can run into the thousands of dollars or more.

The costs of basic needs — food, clothing and shelter — can be enormous for preppers as they plan for disaster. Food cannot just be a few cans of beans and rice with some bottled water, but months or even years worth of prepackaged goods. It can include water filtration and purification systems and thousands of gallons of bottled water. Shelter may translate to a defendable urban shelter or a remote refuge away from cities. Clothing may be extreme weather gear and may include hazardous material suits, warfare and hunting gear. Backup medicines and first aid kits are also basic needs.

For the more extreme survivalists, there is “bugout gear” that goes way beyond a suitcase, backpack and tent. Some preppers keep gold and silver stashes to barter with when paper money will be worthless. Guns, knives and ammunition are also common among preppers. Many go so far as to have generators or at least rudimentary cooking supplies and candles to last for months. Hand-crank emergency radios, specialty tools and equipment are also considered essentials.

Being a doomsday prepper is not actually that new. During the 1950s and 1960s, houses were built with bomb shelters, from the very basic to quite elaborate. Shelters were also not unheard of in the 1970s and 1980s during the height of the Cold War. Congress had a plan to relocate the government to an alternative site in The Greenbrier Resort in White Sulphur Springs, W. Va.

These are the industries making the most money on doomsday preppers.