We exercise and eat healthy to try to stay healthy, so if you get sick, does that mean you slipped up? I’ve written before about why we get colds, but today I’ll tackle a bigger topic: if somebody gets cancer, say, or heart disease, does that mean they could and should have prevented it?

First, let’s talk about genetics, the inevitable half of the “nature vs. nurture” trope. Does disease come from our genes, or from our lifestyle? In truth, those aren’t the only two answers. Surgical oncologist David Gorsky writes that breast cancer patients are often baffled by their diagnosis if cancer didn’t seem to run in their family. But only 5 to 10 percent of cancers are genetic, and in the case of breast cancer, only 27 percent come from factors that Cancer Research UK deems “preventable.” That means more than half have no known cause. Gorsky writes:

“People—including oncologists—really don’t like the concept of ‘sporadic’ cancer, mainly because humans crave explanation. The default assumption is that everything must happen for a reason and there must be a cause for every disease or cancer.”

We can speak in generalities, but that still doesn’t tell us whether one particular person got their cancer from, say, alcohol (one of the preventable causes of breast cancer) rather than from their genes, or just bad luck, or a combination.

Cancer Research UK has compiled some great visuals showing how much cancer is considered to be preventable. Smoking explains 86 percent of lung cancers, for example, while we have little or no control over brain, bone, and prostate cancers.

Exercise, they say, could prevent 3 percent of breast cancers and 3 percent of colon cancers, but doesn’t have an effect on others. Diet has more of an effect, but only on some cancers: lung, bowel, esophagus, stomach, and mouth cancers can be partially prevented by diet, mainly by eating more fruits and vegetables.

Obesity is also very difficult to pin down. Somewhere around 75 percent of the variation in weight from person to person is said to be heritable, but that doesn’t directly translate to the question we’re trying to ask--how many overweight people can blame their genes. We know obesity varies by race, income, and location, and that there are dozens of genes that contribute. We also know that almost anybody can gain or lose weight in the right circumstances. But none of this tells us why any individual is overweight.

What can we do about it?

Perhaps it’s more useful to ask how much control an individual has. In the case of obesity, some 95 percent of dieters gain the weight back within two years. The exact number has been disputed, but clearly very few people maintain weight loss long term.

How much control do we have over other conditions? A good diet can lower blood pressure by a handful of systolic points, which isn’t enough to bring someone from high risk to normal, but could bump them down a level if they’re on the borderline. The CDC estimates that one third of deaths from heart disease are preventable. For type 2 diabetes, a lifestyle intervention that included exercise, a low-fat diet, and weight loss lowered risk by about 58 percent.

But the bottom line is this: even the most preventable chronic diseases are not 100 percent preventable.

Healthy living can reduce risks, but can’t come anywhere near eliminating them. It’s easier to see a cause for lung cancer, which is considered one of the most preventable cancers: 86 percent of lung cancers are considered to be caused by smoking. Still, non-smokers can get it, and there must be a few smokers who got it because of genetics or bad luck. We can speak broadly about “cancers” or “lung cancers,” but that’s no good when looking a friend in the eye: What caused your cancer? There’s no way to know.

Can we simultaneously work to be healthy (eating things that should lower our risk of cancer or heart disease, for example) while being compassionate to those who end up with disease? I say sure. We can’t know the cause of any individual’s disease. And even if somebody did get lung cancer because they smoked, or colon cancer because they didn’t eat a single vegetable their whole life, aren’t they human anyway? Does any good come from making them feel bad about their illness? Have you ever seen somebody being nice to a cancer patient and thought “Hmm, that means cancer must be great, I’ll go have another cigarette now”? No. No, you haven’t.