ABC news anchor Peter Jennings died of lung cancer yesterday at the age of 67.

Jennings learned of his lung cancer diagnosis four months ago. The cancer was reportedly inoperable, meaning that it could not be treated adequately with surgery.

"As you all know, Peter learned only this spring that the health problem he'd been struggling with was lung cancer," said ABC News President David Westin in a statement.

"With Kayce [his wife], he moved straight into an aggressive chemotherapy treatment. He knew that it was an uphill struggle. But he faced it with realism, courage, and a firm hope that he would be one of the fortunate ones. In the end, he was not."

WebMD turned to the medical experts at MedicineNet.com, a WebMD company, for more information about lung cancer.

How common is lung cancer?

Lung cancer is responsible for the most cancer deaths in both men and women throughout the world. The American Cancer Society says about 174,000 new cases of lung cancer in the U.S. were diagnosed in 2004 with more than 160,000 deaths. Lung cancer was not common before the 1930s but increased dramatically over the following decades as tobacco smoking increased.

What causes lung cancer?

The most common cause of lung cancer is smoking, with about 90 percent of lung cancers arising from tobacco use. The risk of lung cancer increases with the number of cigarettes smoked over time.

Pipe and cigar smoking can also cause lung cancer, although the risk is not as high as with cigarette smoking.

Other causes include:

—Passive smoking, or inhalation of tobacco smoke from other smokers sharing living or working quarters. Nonsmokers who live with a smoker have a 24 percent increase in risk for developing lung cancer when compared with other nonsmokers. An estimated 3,000 lung cancer deaths occur each year in the U.S. are due to passive smoking.

—Asbestos fibers. Today, asbestos use is limited or banned in many countries including the Unites States. Both lung cancer and a type of cancer of the lining of the lung called mesothelioma are associated with exposure to asbestos. Cigarette smoking drastically increases the chance of developing an asbestos-related lung cancer in exposed workers. Asbestos workers who do not smoke have a fivefold greater risk of developing lung cancer than nonsmokers, and those asbestos workers who smoke have a risk that is 50 to 90 times greater than nonsmokers.

—Radon gas. Radon gas is a natural, chemically inert gas that is a natural decay product of uranium. It decays to form products that emit a type of ionizing radiation. Radon gas causes 15,000 to 22,000 lung cancer-related deaths annually in the U.S. As with asbestos exposure, smoking greatly increases the risk of lung cancer with radon exposure. Radon gas can travel up through soil and enter homes through gaps in the foundation, pipes, drains, or other openings.

The U.S. Environmental Protection Agency estimates that one out of every 15 homes in the U.S. contains dangerous levels of radon gas. Radon gas is invisible and odorless but can be detected with simple test kits.

—Familial predisposition. While the majority of lung cancers are associated with tobacco smoking, the fact that not all smokers eventually develop lung cancer suggests that other factors, such as individual genetic susceptibility, may play a role in the causation of lung cancer. Numerous studies have shown that lung cancer is more likely to occur in both smoking and nonsmoking relatives of those who have had lung cancer than in the general population.

—Air pollution. Air pollution from vehicles, industry, and power plants can raise the likelihood of developing lung cancer in exposed individuals. Up to 1 percent of lung cancer deaths are attributable to breathing polluted air.

What are the symptoms of lung cancer?

In up to 25 percent of people who get lung cancer, the person does not complain of any symptoms and the cancer is first discovered on a chest X-ray or CT scan. When symptoms are present, the most common symptoms are cough, shortness of breath, wheezing, chest pain, and coughing up blood.

How is lung cancer treated?

Treatment for lung cancer can involve surgery to remove the tumor, chemotherapy, or radiation therapy, as well as combinations of these methods. The decision about which treatments will be appropriate are based on if and where the tumor has spread and the overall health status of the patient.

If the lung cancer is diagnosed early, the goal of treatment is to cure the cancer. If it has spread to other places in the body, this is more difficult to achieve.

What is the prognosis of lung cancer?

The prognosis depends on the type of lung cancer, if and where the lung cancer has spread, and the overall health status of the patient.

The most aggressive form of lung cancer is called small cell lung cancer. Because small cell lung cancer has usually spread by the time it is diagnosed, surgery is not usually helpful. However, small cell lung cancer is also the type of lung cancer most responsive to radiation therapy and chemotherapy. Of all patients with this type of lung cancer, only 5-10 percent are alive five years after diagnosis.

For the other group of lung cancers, called nonsmall cell lung cancer, prognosis varies widely based on if and how far the cancer has spread. In early stages of this lung cancer, 75 percent of people are still alive five years after diagnosis. In advanced-stage disease, chemotherapy offers modest improvements in survival time, although overall survival rates are poor.

Survival rates for lung cancer are generally lower than those for most cancers.

By Michael Smith, MD, reviewed by Brunilda Nazario, MD

SOURCES: Poynter Institute. News release, ABC. MedicineNet.com, a WebMD company. Statement, ABC News President David Westin.