eHealth, a company that helps people buy health insurance plans, is one of the most successful stocks on the S&P 1500.As of September, the company had the second-highest 12-month return on the index, at 162%.Over the last few years, the 22-year-old company turned its attention toward the booming business of Medicare Advantage, the private part of the federally funded government health program for Americans 65 and up. eHealth is aiming to have between $365 and $385 million in revenue by the end of 2019, about a 50% increase over 2018. The company is expecting to hit $1 billion in revenue by 2023.Click here for more BI Prime stories.One of the most successful stocks on the market is eHealth, a company that helps connect people - especially seniors - with health insurance plans. As of September 10, eHealth was up 162% over the past 12 months, the 2nd biggest gain among all the stocks in the S&P 1500 index, which includes most of the US stock market. The stock trades at $72.14.Driving the growth over the last few years has been CEO Scott Flanders' decision to focus the company on the booming market for private health insurance plans for seniors, known as Medicare Advantage, after stumbles selling individual health insurance plans to younger people. EHealth makes a commission over the lifetime of $983 for a Medicare Advantage member the company signs up through its platform as of the second quarter of 2019. That's roughly six times the $167 the company makes over the lifetime of selling a individual health plan to a member.They saw the opportunity there, and they started focusing more on Medicare Advantage, Credit Suisse analyst Jailendra Singh told Business Insider. Singh has an outperform rating on eHealth.The company's been growing its revenue quickly. This year, eHealth expects its total revenue to be between $365 million and $385 million. That's up 50% at the midway point from the $251 million in revenue the company made in 2018. The company has plans to hit $1 billion in revenues by 2023, as the US population ages and more seniors sign up for private Medicare Advantage plans.A changing online insurance business The company got its start more than two decades ago in 1997, founded by a Stanford graduate with the idea of bringing health insurance online.The business had been chugging along, getting a sizeable chunk of the individual market. eHealth went public in 2006, and at its peak in 2014 had about 800,000 members signed on, Flanders said.But in 2010, the Affordable Care Act threw a wrench in eHealth plans. The health law created new federal and state-run markets for health insurance plans, taking away much of eHealth's business. Health insurance sales under the ACA began in 2013.Just 29,698 people used eHealth to buy individual health insurance plans by the end of 2018. In 2016, eHealth's board hired on Scott Flanders to start turning things around. Flanders had served as a board member since 2008, and prior to joining eHealth, he had been CEO of Playboy Enterprises, the media and lifestyle group that publishes Playboy magazine. He previously led a media company, and worked in the music and video distribution business.Stepping into the job, Flanders faced a big challenges: he had to get a grasp on the healthcare industry, months before the 2016 presidential election.I never remember being less certain on my feet than having stepped into this, Flanders said. Getting an understanding of how all the pieces are connected and what each group's agenda was a challenge, he said.Stepping into the role, Flanders doubled down on a growing area for the company, the Medicare Advantage market. That strategy is paying off. Since Flanders was announced as CEO on May 31, 2016, eHealth's stock is up 422%, and the company now has a market value of $1.8 billion. The S&P 1500 index has gained 41.3% over the same time period.The market for Medicare Advantage plans is competitive. About a third of people on Medicare are enrolled in private Medicare Advantage health plans.People can typically choose to enroll in Traditional Medicare or Medicare Advantage plans when they turn 65. Either way, their health needs are largely funded by the US government.As of last year, more than 20 million Americans were enrolled in Medicare Advantage plans. It's a growing market that insurers from startups like Oscar Health, Clover Health, Devoted Health, and Bright Health are interested in getting a piece of, alongside entrenched rivals like Humana, UnitedHealth Group, and CVS Health are in. How eHealth has picked up a piece of the growing Medicare Advantage marketAs the competition among insurers to sign on more Medicare Advantage members heats up, insurers turn to brokers including organizations like eHealth to get their plans in front of more people. We help reach seniors that the carriers aren't reaching with their own traditional marketing efforts, Flanders said. Humana and UnitedHealthcare make up a combined 40% of eHealth's revenue as of 2018, while Aetna made up 14%, according to eHealth's annual report. So far, Flanders said, eHealth has its hand on about 1% of the Medicare Advantage brokering market. Roughly 80% of the deals today happen via phone, with the remaining 20% picking plans through eHealth's website. eHealth had signed up 521,000 Medicare Advantage members as of June 2019. The stock has turned down over the past month, amid worries that support is growing among Democrats for a national health-insurance plan known as 'Medicare for All,' according to analysts at Evercore ISI. They also said that CMS is working on updates to its Medicare Plan Finder tool that could take some business from eHealth. For eHealth, the hope is to keep growing as more Americans turn 65 and opt for the private part of Medicare Advantage. As such, eHealth has plans to get to a $1 billion in revenue by 2023, nearly four times what the company made in revenue in 2018. Singh said that so far, the past few years' results suggest eHealth could get there. Their results have shown they're headed in the right direction, Singh said.