Smoking changes a person's gene activity across the body, thus raising the risk of developing various diseases, a new study, published Thursday in the journal Medical Genomics, suggests.

The findings may be a clue to why smoking affects overall health -- from heart disease to combating infections, according to the study conducted by a team of researchers from San Antonio, Texas and the Menzies Research Institute Tasmania in Australia.

The researchers analyzed blood cell samples of 1,240 people, ages 16-94, who were participating in the San Antonio Family Heart Study.

They found that the self-identified smokers in the group -- 297 people -- were more likely to have unusual patterns of "gene expression" related to tumor development, inflammation, virus elimination, cell death and more. A gene is expressed when it codes for a protein that then instructs, or kick-starts, a process in the body.

Cigarette smoking could increase or decrease the level of expression of 323 genes, according to the study.

"On some levels, we were surprised by the extent of the influence exposure to cigarette smoke had on gene expression, especially considering we used such a simple measure of smoke exposure: smoker or non-smoker," said lead author Jac Charlesworth, a research fellow at the Menzies Research Institute Tasmania in Australia.

On the other hand, Charlesworth said, doctors have known "for a very long time" that smoking worsens cancer risk overall, depresses immune systems and causes other problems like heart disease, cataracts and poor wound healing.

"One of the most important things this study did was to go some way towards understanding the biological responses the body has to smoking," Charlesworth said. "Now we know that part of the process is the result of changes at the gene level."

Cigarette smoke contains more than 4,000 chemical compounds, some of them known toxins and carcinogens. When someone smokes, the toxins enter the bloodstream through the lungs and are distributed throughout the body, according to the study.

Because the activity of a single gene may influence a whole set of other genes' expression, the study could not explain which chemical in cigarette smoke was responsible for affecting which gene.

"Our results indicate that not only individual genes but entire networks of gene interaction are influenced by cigarette smoking," the study noted.

But the researchers said further study is needed.