You've heard it a thousand times: Stress is bad for you. It can speed your heart rate and constrict blood vessels, leading to high blood pressure and heart disease. It can interfere with your sex drive, cause indigestion and give you headaches. All of that is bad enough, but disturbing new research shows that stress can also cause negative and long-term changes to your immune system, leaving you more vulnerable to a host of chronic diseases, even some deadly ones.

Scientists in Israel recently published their research in a fascinating branch of study that's grown very popular of late--the relationship between the bacteria in your gut and pretty much every other organ and system in your body. Could emotional or social stress affect those bacteria, they wondered

To find out, they worked with two groups of groups of mice in tightly controlled lab conditions. In one group, the mice simply hung around in their cages for 10 days. Members of the other group were placed for five minutes at a time in the cage of an aggressive, territorial, dominant mouse, which of course attacked them (if they were actually injured, they were removed from the experiment). The attacked mice then spent the next 24 hours behind a clear divider where they could see their aggressor and receive "sensual cues" so that they would be frightened but not exposed to harm.

In short, these were some badly stressed-out mice. After 10 days, the researchers examined the bacteria in their gut. Compared to the control group, there was an increase in two types of bacteria in particular, bilophila and dehalobacterium, both of which are also appear in higher quantities in humans suffering from autoimmune diseases, particularly multiple sclerosis.

And it gets worse. Researchers examined the bacteria themselves and discovered that their DNA had altered, making them more aggressive. They were now better able to travel through their mouse hosts into the lymph system, triggering the immune system to become overactive and turn against the body, which is how autoimmune diseases takes root. Besides MS, these include type 1 diabetes, Grave's disease, Guillain-Barre syndrome, and many others. Most of these are serious, long-term illnesses with treatments that include things like surgical removal of the thyroid or filtering of the blood. 

A long-term effect.

The effect of stress on gut bacteria seems to be reversible, but the damage to the immune system may be permanent. When researchers stopped stressing the mice, their bacterial load returned to normal levels after 14 days (which is a long time for a mouse). But the damage to the immune system was not undone and the bacteria that had spread to the lymph glands did not go away, something the researchers called a "hit-and-run-effect." Their results, they say, suggest that a stress event can have consequences for your body's function even many years later.

None of us live a stress-free life. And all of us go through hard times at some time or other. Like the hapless mice, some of us may even find ourselves under attack by a powerful and aggressive rival. But if you're under severe stress, you can't ignore it, and you can't tell yourself it doesn't matter, because it does. You may think you can just handle it--and perhaps you can--but the effects on your health could be real, and very long-lasting. Removing the source of the stress, or doing your best to counteract it should jump to the top of your priorities list. Otherwise, you could find yourself paying the price for years to come.