What industry should experience solid demand even if the economy takes a downturn? What industry represents nearly one-fifth of the U.S. economy and is growing rapidly? What industry should experience growing demand over the long run because of unstoppable demographic trends? The answers: healthcare, healthcare, and healthcare.

These questions point to several top reasons to invest in healthcare. But where to invest? Here's why Alexion Pharmaceuticals (NASDAQ:ALXN), Edwards Lifesciences (NYSE:EW), and Mylan N.V. (NASDAQ:MYL) are three of the top healthcare stocks you can buy right now.

A finger touches a button with a healthcare icon.

Image source: Getty Images.

Alexion Pharmaceuticals: A winner in treating ultra-rare diseases

Alexion rose to prominence with Soliris. The drug won U.S. regulatory approval in in 2007 for treating paroxysmal nocturnal hemoglobinuria (PNH) and in 2011 for treating atypical hemolytic uremic syndrome (aHUS). Both PNH and aHUS are extremely rare diseases. Despite the low numbers of patients worldwide with PNH and aHUS, sales for Soliris topped $2.8 billion last year -- in large part because it's one of the most expensive drugs in the world

The biotech has several pathways to grow revenue and earnings more in the future. Alexion claims two other drugs on the market with solid potential. Strensiq treats ultra-rare genetic disease hypophosphatasia. Kenuma treats another ultra-rare disease, lysosomal acid lipase deficiency.

Alexion is evaluating Soliris in two late-stage clinical studies for treating a couple of other rare diseases -- generalized myasthenia gravis and neuromyelitis optica spectrum disorder. The company's pipeline also includes two other late-stage candidates: ALXN1210 is targeting treatment of PNH and AHUS, while ALXN1101 is being evaluated for treating molybdenum cofactor deficiency type A, an ultra-rare metabolic disorder.

As a result of all of these opportunities, Wall Street analysts are pretty bullish on Alexion's prospects. The biotech's earnings are expected to grow nearly 19% annually over the next few years.

Edwards Lifesciences: A pioneer in heart valve therapies

Edwards Lifesciences is a pioneer in the development of heart valve therapies. The company's Sapien transcatheter aortic heart valves are used in patients for whom traditional open-heart surgery isn't ideal. Edwards also develops and markets surgical heart valves and hemodynamic monitoring systems used to measure a patient's heart function and fluid status in surgical and intensive care settings.

It's fair to say that Edwards Lifesciences dominates the markets it serves. Over 95% of the company's revenue comes from products that rank No. 1 globally in their respective markets. Edwards should be able to continue its leadership position through continued innovation and acquisitions.

The transcatheter heart valve market is the primary opportunity for Edwards Lifesciences to grow. The company expects this market to double by 2021, driven by three primary factors: expansion of indications, technology advances, and making patients more aware of the advantages of transcatheter hear valves. Even more growth is possible beyond 2021 as Edwards expands more internationally. Another important growth driver for Edwards Lifesciences is the aging global population, which should cause the numbers of patients with structural heart disorders to increase.

Edwards Lifesciences has grown its earnings by solid double-digit percentages over the last five years. Analysts expect the same kind of growth in the future, with consensus estimates calling for average annual earnings growth of 15% over the next few years, making Edwards one of the best medical-device stocks around for growth investors

Mylan: A bargain drug stock

Mylan is a major global pharmaceutical company that develops and markets generic, over the counter, and brand drugs. The company ranks No. 6 worldwide in overall prescription volume thanks to its more than 7,500 products on the market and operations spanning over 165 countries and territories.

Last year wasn't the greatest for Mylan. The drugmaker received a lot of negative publicity after it jacked up the prices of its EpiPen auto-injector. Mylan also had to pay $465 million in a settlement with the U.S. Department of Justice and other government agencies over allegations that it improperly classified EpiPen products so it could pay lower rebates to Medicaid. 

Mylan probably won't achieve the kind of growth that Alexion or Edwards Lifesciences will. Much of the company's growth potential stems from its acquisition last year of Swedish drugmaker Meda. This acquisition is driving revenue higher in Europe and throughout the rest of the world. However, Mylan faces headwinds in North America with the launch of the generic version of EpiPen. Wall Street analysts project average annual earnings growth of around 9%.

So why buy Mylan? The controversies from 2016 drove the stock down, and it now looks quite attractive. Mylan's shares trade at less than seven times expected earnings. With its decent growth prospects and low valuation, Mylan should be a winner for investors over the long run. 

Keith Speights has no position in any stocks mentioned. The Motley Fool recommends Mylan. The Motley Fool has a disclosure policy.