(CNN) -- The man who dared to boldly go where no one had gone before has revealed that he can't go as many places as he used to.
"I'm doing OK," Leonard Nimoy tweeted Friday. "Just can't walk distances. Love my life, family, friends and followers."
Nimoy announced last week that he has been diagnosed with chronic obstructive pulmonary disease -- 30 years after having given up cigarettes.
"Not soon enough," tweeted the 82-year-old actor and director who played Spock, the half-human science officer aboard the Starship Enterprise from 1966 to 1969 on the TV series "Star Trek" and in movies during the decades since. "I have COPD. Grandpa says, quit now!!! LLAP."
The last reference is to his signature phrase, "Live long and prosper."
Nimoy, who told an interviewer last year that he flunked chemistry in high school, may not have grasped the long-term risks associated with smoking, but his announcement is not surprising, said Dr. Richard Casaburi, a pulmonologist at Los Angeles Biomedical Research Institute in Torrance, California.
Smoking could very well be responsible for the appearance of symptoms of COPD decades after quitting, because it is a progressive disease and lung function declines with age, said Casaburi, who is not involved with Nimoy's treatment.
COPD is the third-leading cause of death in the United States -- after heart disease and cancer -- and smoking is responsible for the vast majority of cases, he said.
Some 12 million people in the United States have been diagnosed with the disease, and perhaps another 12 million have it, but don't know, he said, adding that about 30% of people who smoke will get clinically significant COPD. "The mystery really is why everybody doesn't get it."
Initial symptoms can include cough, sputum production and shortness of breath, especially on exertion, which limits their activity.
But medications typically can ease the symptoms, he said. "It's a disease of the older person, but the damage is started when they're young."
Though smoking rates have declined in the United States in recent years, COPD takes years to develop, so the incidence is rising, said the 66-year-old doctor. "We're now seeing the results of people my age who started smoking in the 1960s and now are reaping the consequences."
He predicted the incidence will decline sharply over the next 20 to 30 years.
More severe symptoms can require oxygen therapy and may lead to pneumonia and heart failure, said Dr. Norman Edelman, senior medical consultant to the American Lung Association.
He praised Nimoy for his announcement and his admonition to stay away from tobacco. "We're delighted," Edelman said in a telephone interview. "The more people speak out, the more ammunition we have to fight -- for not only smoking cessation but for research that will lead to better treatment of COPD."
Funding has been limited by the belief among some that patients have only themselves to blame, a point of view that is unfair, he said. "These people deserve the best treatment that we can provide," he said, adding that most smokers take up the habit before they are adults.
But Nimoy's illness has not interfered with his goal of living long and prospering. In 2010, life expectancy at birth for someone born in the United States reached 78.7, and Nimoy, who is 82, appears to be continuing to prosper, too. "Just taped my comments for Star Trek Fest to be aired on EPIX Feb. 16th," he wrote. "Still enjoying my life. LLAP."