Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you expect the medical equipment industry to thrive as the world's population grows and ages, the SPDR S&P Health Care Equipment (NYSE: XHE) ETF could save you a lot of trouble. Instead of trying to figure out which companies will perform best, you can use this ETF to invest in several dozen of them simultaneously.

The basics
ETFs often sport lower expense ratios than their mutual fund cousins. The health-care equipment ETF's expense ratio -- its annual fee -- is a low 0.35%.

This ETF is new, having started in late January, so there isn't much of a track record to assess yet. It's encouraging that its peer, the iShares Dow Jones US Medical Devices (NYSE: IHI) ETF, outperformed the S&P 500 over the past three and five years, but although they hold many of the same stocks, the two ETFs won't perform in lockstep.

As with most investments, we can't expect outstanding performances in every quarter or year. Investors with conviction need to wait for their holdings to deliver. With a low turnover rate of 25%, this fund isn't frantically and frequently rejiggering its holdings, as many funds do.

What's in it?
Several of this ETF's components have the promise to make strong contributions to its performance in the future. Intuitive Surgical (Nasdaq: ISRG), which specializes in robotic surgical equipment, has been a market darling for several years now. Some Fools think it will be one of the greatest companies of 2020. It's risen 9% since the end of January. Hologic (Nasdaq: HOLX), sporting a high "Buffett Ratio," gained 37% as it develops medical equipment for women, including mammography, cervical cancer screening, gynecological exams, and bone density assessments. Its stock is up about 3%.

Spun off from Baxter International (NYSE: BAX) in 2009, Edwards Lifesciences (NYSE: EW) is up about 1% since the end of January. If its Sapien heart valve technology is cleared for use in the U.S., the company's outlook will be even brighter. Boston Scientific (NYSE: BSX) also gained 1%, despite dealing with fallout from its regrettable purchase of Guidant, which has led to lawsuits and costly settlements.

The big picture
Demand for medical devices isn't going away anytime soon. A well-chosen ETF can grant you instant diversification across the industry -- and make investing in and profiting from the sector that much easier.