It's a nice sunny day out in the wild, where a hunter-gatherer man is enjoying his dinner of minimally processed plants and meat. Surrounding the hunter-gatherer is a hoard of worms, parasites, and bacteria--organisms with which he has shared a home since his birth. This is the environment, many scientists now argue, to which the modern day human is adapted.

If you fast forward to the 21st century, however, our Paleolithic bodies are living in a very different modern world. Gone are the days of hunting and eating game animals and large amounts of wild plants. In the West especially, urbanization and increased standards of hygiene have depleted our environment of the microbes that the human immune system once needed to learn to tolerate. The hunter-gatherer diet has been overwhelmingly replaced by large helpings of grains, refined sugars, vegetable oil, dairy products, cereals, and other processed foods.

Our world has changed quickly. Our genes, not so much. Could the discrepancy between the environment to which humans are adapted and the one in which we now live be making us sick?

According to the World Health Organization, chronic diseases--like heart disease, respiratory disease, diabetes, obesity and stroke--are responsible for 63% of deaths worldwide. Some scientists link this rise in chronic disease to the change from a hunter-gatherer diet to the modern "Western" diet.

Hunter-gatherers took in about one-third of their daily calories from animal meat, usually lean meats and fish, and the remaining two-thirds from fruits, vegetables, nuts, and other plant foods. Today, however, about 70% of the human diet is made up of foods the hunter-gatherer would very rarely have eaten, if they saw them at all: cereals and refined grains, milks, cheeses, syrups and refined sugars, cooking oils, salad dressings, etc.

The hunter-gatherer diet provides very different nutrients from those in the modern diet. Hunter-gatherers took in considerably higher levels of fiber and various vitamins and minerals, including vitamin C, vitamin B-6 and -12, calcium, and zinc. They also took in significantly less sodium. While hunter-gatherers probably ate less than 1,000 milligrams of sodium each day, the average American consumes three times that amount.

Why should it matter that we eat differently than humans did tens of thousands of years ago? Were hunter-gatherers actually healthier than we are now? S. Boyd Eaton, M.D, of Emory University in Atlanta, Georgia, believes so. All of these diet differences may adversely affect health. High sodium intake, for example, can cause hypertension or osteoporosis, while lower protein consumption could cause stroke or weight gain. "Human ancestors had almost no heart disease" and no obesity, Eaton argues.

Yet critics point out that our ancestors simply did not live long enough to experience chronic disease. Since there is little age-related information about very early humans, their life expectancy is estimated using statistics from the 18th century and from current hunter-gatherers. This evidence suggests that the average life expectancy of pre-industrial humans was probably 30-35, whereas now, the average human life expectancy is about 68 years.

Eaton, however, argues that the physical signs of cardiovascular disease can be detected much earlier in life. For instance, when Dr. Abraham Joseph and his colleagues at the University of Louisville looked at trauma victims aged 14 through 35, they found that about three-quarters of them already had evidence of cardiovascular disease.

And it's not just our diet that is changing faster than our genes can keep up. As cities were built and hygiene standards increased, the critters with which we once needed to adapt to live have been wiped from our environments. Some researchers believe our bodies miss these worms and bugs and that disorders like allergies, asthma, and inflammatory bowel disease could be the result.

"All of these organisms that you pick up everyday have to be tolerated," explains Dr. Graham Rook of University College London. When parasites are ubiquitous, as in Paleolithic times, they "start to be relied on to regulate the immune system." When we don't encounter these critters, Rook hypothesizes, it upsets a delicate balance between immune cells in the human body.

In fact, epidemiological studies have shown a correlation between hygiene and the prevalence of inflammatory and immune diseases. In undeveloped countries, where there are still high populations of the worms and bugs with which humans developed, inflammatory conditions are less common, says Dr. Joel Weinstock, chief of gastroenterology and hepatology at Tufts Medical Center. Weinstock is currently investigating how parasitic worms interact with the immune systems of their human hosts.

Some doctors are now hoping these "old friends" could inform new treatments for allergies and other immune disorders. Two such researchers, Dr. Jorge Correale and Dr. Mauricio Farez, of the Raúl Carrea Institute for Neurological Research in Argentina, performed a study in 2007 in which they infected multiple sclerosis (MS) patients with parasites. The researchers hoped to determine whether parasite infection could reduce symptoms of MS, an autoimmune disease, which include numbness, muscle weakness and spasms, vision loss, problems walking, and speech problems.

Patients in the study were randomly assigned to receive treatment with one of four different species of worms. Uninfected MS patients and healthy control individuals were used as comparisons. The researchers found that during the almost five year follow-up period, MS patients infected with worms showed significantly fewer flare-ups than non-infected MS patients. Plus, infected patients produced more of the particular immune cells that regulate the immune system--the same cells that Rook and others believe have declined due to the increased cleanliness of our living spaces.

Rook admits that our "unnaturally" hygienic environment is probably just one factor in the dramatic rise of autoimmune and inflammatory disease. Dr. Scott Weiss of the Harvard University School of Medicine has another culprit in mind, particularly when it comes to asthma: vitamin D deficiency. "If you really look at what has happened with autoimmune disease," he says, "they started to increase in the 1950s and even more dramatically in the '60s, '70s, and '80s. Now they have leveled off." This is the same time period, Weiss explains, during which we started spending less time outdoors and more time inside, enjoying new inventions like television and air conditioning. Since sunlight triggers the body's natural production of vitamin D, less time outside in the sun means less vitamin D for the body.

Weiss and his colleagues found that women who took in more vitamin D when pregnant delivered babies who were less likely to have asthma-related symptoms, like wheezing, as toddlers. Our hunter-gatherer ancestors probably also saw less asthma because they spent most of their time outside, absorbing all that sunny, vitamin D.

Is it time, then, to give up your television, air conditioning, processed food and Purell and head to the sunny outdoors to kill and scavenge your own meals, hunter-gatherer style? Well, maybe not yet--at least not completely. As Rook explains, relaxing hygiene now wouldn't help us get back our old friends, but instead would expose us to new dangers. While there may be benefits to our old hunter-gatherer diets, our current diet has its advantages, like the higher calorie content.

Nevertheless, we live in a world that is very different from the one in which our ancestors evolved. Our genes have not changed as quickly as our diet, physical environment and lifestyle. Until our genes can catch up to the world we've created, we may have to find ways to bring back pieces of our old world.