A new study reveals that HIV patients who are smoking are more likely to die from smoke-related illnesses than the virus.

The study, published in the Journal of Infectious Diseases, showed that HIV patients who aged 40 and above and continue to smoke even after entering treatment lose about 6.7 years from their life expectancy.

"It is well known that smoking is bad for health, but we demonstrate in this study just how bad it is," explained co-author Dr. Krishna Reddy of Massachusetts General Hospital and Harvard Medical School in a report from NBC News. "We actually quantify the risk, and I think providing those numbers to patients can help put their own risks from smoking in perspective. A person with HIV who consistently takes HIV medicines but smokes is much more likely to die of a smoking-related disease than of HIV itself."

For the study, the researchers used a computer simulation of HIV disease and treatment to project the life expectancy of HIV-infected persons, based on smoking status. The researchers found out that 40 percent of patients battling with HIV are smokers.

Their simulations also showed that patients who strictly follow their HIV treatment but still smoke have their life expectancy decreased by about twice as much as HIV does. However, patients who quit smoking during the course of their treatment could regain up to 5.7 years of their life.

According to the Centers for Disease Control and Prevention, smokers infected with HIV are most likely to develop thrush, hairy leukoplakia, bacterial pneumonia, Pneumocystis pneumonia. Smoking while battling with HIV could also lead to serious and deadly illnesses, including chronic obstructive pulmonary disease, cancers, heart disease, respiratory disease and stroke.

About one in five Americans smokes cigarettes. Smoking is one of the leading causes of deaths in the United States, with about one in five deaths related to smoking every year.