Researchers call it the "unifying theory" behind the major killers of our times -- cardiovascular disease, cancer and diabetes -- as well as scores of other diseases.

It's inflammation, which most people know by the red, painful swelling that follows an injury, bug bite or other surface wound. But it also exists in tissue far below the skin, and scientists are now convinced this below-the-eye inflammation is the culprit that worsens many chronic diseases.

And while inflammation is the immune system's response for beating back invaders in the body, inflammation gone awry can lead to heart attacks and strokes, aid cancers in turning deadly, cause Alzheimer's disease by destroying brain cells, and usher in diabetes.

Out-of-kilter inflammation is also linked to clinical depression, schizophrenia, Parkinson's disease, asthma, osteoarthritis, liver disease and hypertension, among others disorders.

"Retrospectively, it's what we should have been looking it," said Lisa Coussens, a cancer biologist at the University of California, San Francisco. "It's a major area of research."

Coussens and several other medical experts spoke last week at a conference sponsored by the University of California, San Francisco, that gave its audience a snapshot of research to date on the connections between inflammation and chronic diseases.

The speakers emphasized the promise inherent in the remarkable realization that one condition links so many diseases: Fundamentally similar treatments could be used to control them.

Some of the best evidence to date of inflammation's crucial role in exacerbating chronic diseases comes from studies on cancer rates in patients who were counseled to lower their risk for a heart attack by regularly taking anti-inflammatory medicines like aspirin. That population has a significantly lower rate of cancer, compared to a similar population not taking anti-inflammatory medicines, Coussens pointed out.

"They were compelling," Coussens said.

As another example of the beneficial effect of controlling inflammation with over-the-counter medicines, regular use of ibuprofen is associated with lowered rates of Alzheimer's disease, noted Cynthia Lemere, a professor of neurology at Harvard Medical School, who spoke at the San Francisco conference.

Coussens emphasized, however, that long-term use of aspirin and other anti-inflammatory drugs may cause other severe health complications or dangerous drug interactions, and a physician should be consulted before taking any medicine on a regular basis.

The growing knowledge of chronic inflammation's underlying role in so many diseases gives added urgency to the well-known advice to maintain a healthful lifestyle, including getting adequate exercise and sleep, maintaining a balanced diet and avoiding undue stress.

Deficits in nutrition and sleep are linked to increased inflammation, as is excess stress. Among other benefits, exercise wards off weight gain, which can also triggers chronic inflammation.

And once a disease reaches a serious stage, there's far less optimism among scientists that controlling inflammation can undo the damage.

"We're all talking about prevention, not reversal," said Lemere.

But there's also excitement among researchers that if a disease is caught early enough, then taming inflammation could provide an effective tool for stopping the disease's advance.

Chronic inflammation is caused by the persistent presence of pathogens and environmental pollutants, lifestyle and genetics, among others factors.

When the immune system is functioning normally, its front line fighters swiftly respond to any injury, such as a splinter puncturing the skin, or an invasion of microbes from a scrape or bug bite. These thug-like warriors, which go by names like natural killer cells and macrophages, shoot out an arsenal of chemicals, rip holes into cell walls of invaders, or even consume them whole.

But these fighters, part of the more primitive "innate immune system," lack the discriminate nature of their more sophisticated immune system counterparts, called the adaptive immune system. But the latter takes longer to mobilize before sending out its more targeted arsenal.

In the meantime, these brutish cellular warriors sometimes take out healthy cells, a form of collateral damage. (The more sophisticated fighters aren't without their flaws -- they too can damage healthy cells.)

An immune system that's off-kilter is rarely the direct cause of a chronic disease, researchers emphasize. Instead, it can perform a devastating secondary role.

In the case of Alzheimer's disease, explained Lemere, the Harvard neurologist, an innate immune cell works to ingest a type of plaque outside brain cells of people with the disease.

But sometimes these immune cells ineffectively resort to chemical warfare against the plaque, which then destroy nearby brain cells. It's this brain cell death that causes the symptoms of Alzheimer's disease, including severe memory loss and mood swings.

"I don't consider inflammation the cause of the disease," said Lemere. "But it certainly drives it forward. (The immune cells) are initially trying to help, but they get disregulated. They get revved up. It's sort of this big, vicious feedback loop."

Innate immune cells can cause cardiovascular disease by ingesting "bad cholesterol" that's begun to literally go rancid. These immune cells can then get trapped in arterial walls, and contribute to plaque build up. They may also stay on the attack, causing chronic inflammation. The plaque can then break off due to inflammation, and form a blood clot that triggers a heart attack or stroke.

With Type 2 diabetes, molecules released by both innate immune cells and fat cells are implicated in interfering with the normal function of insulin, which regulates blood sugar levels.

When immune cells drop their armor after detecting danger has passed, they take on a healing role by secreting substances that promote blood vessel and tissue growth.

But tumor cells can send signals that confuse these immune cells, causing them to switch allegiance and aid and abet tumor growth instead of normal tissue growth. Tumors, like any other tissue, need a supply of oxygen and nutrients that's provided by blood.

Most cancers don't become deadly until they spread, or metastasize. Researchers believe it's these hijacked immune cells that foster the spread of cancer. And if caught before the tumor has spread, anti-inflammatory treatment may be able to contain the tumor, creating the prospect of cancer becoming a manageable disease.

"That's precisely the point," said Coussens.