Having a stock double in value is a good thing. Having it double—or more—in a year is even better. Keith Speights, writing for The Motley Fool, looks at three healthcare stocks that have doubled in less than 12 months.

1. Align Technology

Based in San Jose, California, Align Technology is a medical device company in the orthodontic area. It manufactures and markets Invisalign clear aligners (for teeth), iTero Intraoral scanners, and OrthoCAD digital services for dental professionals.

The company was ranked at the top S&P 500 stock of 2017, growing more than 190 percent over the last 12 months. The Invisalign aligners are like metal-and-wire braces for teeth, but they’re almost transparent.

"But it’s not just Invisalign that’s making Align successful," Speights writes. "The company partners with SmileDirectClub to provide non-Invisalign clear aligners that ship directly to patients at home. Align also markets a lineup of intra-oral scanners that dental professional use to create 3D images of patients’ teeth. Align then uses these images to create treatment plans for Invisalign. With all of these products continuing to pick up momentum, Align appears to be on track for sustained strong growth in 2018."

Align Technology stock is currently trading for $276.39.

2. Bluebird bio

Headquartered in Cambridge, Mass., bluebird bio is a clinical-stage biotech company focused on gene therapies and T-cell-based immuno-oncology therapies. On Jan. 8, the company announced it had issued an additional 277,109 shares of common stock at the public offering price of $185 per share, to raise $51.3 million.

At the American Society of Hematology Annual Meeting in December 2017, bluebird presented data on its LentiGlobin gene therapy in severe sickle cell disease and transfusion-dependent beta-thalassemia. Dave Davison, the company’s chief medical officer, said in a statement, "The durable treatment effects observed to date in this study are encouraging, particularly given the manufacturing process improvements that we implemented across our subsequent clinical studies of LentiGlobin, and additional changes to the HGB-206 study protocol that we hope will further improve outcomes for patients with SCD."

Shares almost tripled over the last 12 months. The big jump was in June after it reported positive early-stage data from its CAR-T therapy bb2121 for multiple myeloma, which it collaborates on with Celgene.

"That relationship with Celgene appears to be paying off for bluebird in several ways," Speights writes. "Bluebird has received financial assistance in advancing its pipeline products. And, especially over the past week, bluebird stock has enjoyed a nice boost thanks to reports, then confirmation, that Celgene was buying Juno Therapeutics. Like bluebird, Juno also has a CAR-T program. The possibility that Celgene could acquire bluebird as well has captured the imaginations of many investors."

Bluebird bio stocks are currently trading for $195.15.

3. Exact Sciences

Located in Madison, Wisconsin, Exact Sciences manufactures and markets Cologuard, the first and only FDA-approved noninvasive colorectal cancer screening test. Anyone over the age of 50 who’s had a colonoscopy would be delighted if this test became standard of care. The Centers for Medicare and Medicaid Services (CMS) proposed last February, which was later approved, that Cologuard be in the Star Ratings program for Medicare Advantage plans.

"It certainly helped, though, that Exact Sciences simply executed well in 2017," Speights writes. "The company persuaded more payers to cover Cologuard. It sold more physicians on ordering the DNA test. And Exact Sciences did a good job educating patients about the benefits of Cologuard, especially through its TV marketing campaign."

Shares have risen almost 170 percent over the last 12 months.

Exact Sciences shares are currently trading for $9.54.