This can translate into a few hundred dollars a month—a significant sum to people who have few or no other sources of income. In 2015, one of us wrote an article for The Atlantic about people who sell their plasma to make ends meet, and other journalists have focused on them as well. But what’s gotten much less attention is the global, multibillion-dollar industry these donors are sustaining. It’s an industry that helps heal patients worldwide, but it’s also one that, to keep the blood flowing, depends on the existence of a large group of people desperate for cash.

There is a thriving industry built on plasma because the substance has no substitute in medical treatments for certain rare conditions, such as antibody deficiencies and hemophilia. As one company explained in an annual report, plasma comes from “the perfect bioreactor developed over millions of years of evolution: the human body.” In addition, many treatments are so plasma-intensive that they collectively require millions of liters of it each year; a year’s worth of therapy for someone with hemophilia might require the yield of as many as 1,200 donations. So, not only is there no substitute for plasma, but enormous volumes of it are needed. On top of that, global demand for it has steadily risen as many countries’ populations age and need more treatments that require plasma.

This all has been a boon for the industry. Over a decade, the number of donations—really, “sales” is a more accurate noun—in America tripled, from 12 million per year in 2006 to 38 million per year in 2016, according to the Plasma Protein Therapeutics Association, a trade group. The number of donation centers in the U.S. has more than doubled to meet demand, from fewer than 300 sites in 2005 to over 600 today. Global sales jumped from $5 billion in 2000 to $20 billion in 2015, and are expected to keep growing at a rapid clip well into the next decade.

The Number of Annual Plasma Donations in the U.S., from 1999 to 2016

Data: Plasma Protein Therapeutics Association

Because plasma can be a medical necessity, companies that collect it tend to wield significant pricing power. These days, a liter of plasma that costs a company about $150 to collect and process could sell for in the neighborhood of $500—a substantial markup in any industry.

Because there are high barriers to entering the industry—it’s not easy for an entrepreneur to just up and start a medically sensitive operation—a few international companies command a large chunk of the market share and keep profits high. The industry thrives in the U.S. in particular because the country allows compensation of donors (many countries do not) and has some of the least restrictive plasma regulations in the world. In the United States, a person can donate up to 104 times a year, while in much of Europe, donors are limited to 45 times in a year. Little is known about the long-term health consequences of chronic donation, although a few years ago, Darryl Wellington wrote in The Atlantic about some of the effects he and some other “plassers” experienced, including extreme fatigue and blacking out.