We’ve all heard the advice about avoiding Lyme disease. If you walk through wooded or grassy areas where it’s prevalent, you should use insect repellent. Cover exposed skin. Check yourself thoroughly once you return home, and take a shower. If you see a tick, pluck it off your skin with tweezers. Look out for a bull’s eye-shaped rash and flulike symptoms in the summer.

About 30,000 cases of Lyme disease are reported to the Centers for Disease Control and Prevention each year, making it the most commonly reported vector-borne illness in the United States. That number has tripled over the last 20 years. And experts estimate that the actual number of cases — not just those that happen to be reported to the agency — is more like 300,000 per year.

If Lyme has become so common, why isn’t there a vaccine for it? Well, here’s something you may not know: There used to be one, but it was taken off the market more than 15 years ago. And there’s only one new vaccine candidate in the pipeline.

“Clearly, the problem is getting worse,” said Dr. Paul Mead, a top scientist at the C.D.C. “For years, we have been advocating that people use repellents, do tick checks, spray their yards. That remains good to do, but it’s not enough.”