Industrial agriculture is making us sick. The Centers for Disease Control (CDC), World Health Organization (WHO), nutritional scientists, and medical professionals warn against the health risks of concentrated animal feeding operations (CAFOs). There is a near consensus among experts that overuse of antibiotics, crowded and unsanitary livestock conditions, unnatural feed diets, and a lack of diversification are responsible for some serious global health risks.

Contracting bugs from animals is nothing new. In fact, zoonoses (infectious diseases transferred between species) are a natural part of evolutionary biology. But modern industrial farming practices can turn health issues that were once fairly benign into real concerns.

Factory farming creates perfect conditions for the proliferation of super bugs: The stress and unsanitary conditions of CAFOs weaken animals' immune systems, making them more susceptible to infection; overcrowding allows disease to spread quickly and easily; and over time, antibiotics can cause resistant strains of bacteria to evolve. These conditions, combined with a lack of diversification, create a petri dish for dangerous diseases.

As Dr. Michael Greger, director of public health and animal agriculture for the Humane Society of the United States, writes:

Unnaturally high concentrations of animals confined indoors in a limited airspace and producing significant quantities of waste may allow for the rapid selection, amplification, and dissemination of zoonotic pathogens. [The Human/Animal Interface]

Here are five diseases that have been affected by CAFOs:

1. E. coli

The content of animal feed poses particular health risks. Traditionally, cattle subsisted on a grass-based diet, but government corn subsidies and demand for more fatty, marbled beef motivated farmers to switch over to grain-based feed. Enter E. coli.

Milder strains of the bacteria have always been present in cows' stomachs, but the introduction of a grain-based diet upped the ante, according to a study published by the U.S.'s National Centers for Biotechnology Information in 2009. Cows' digestives systems became more acidic in order to tolerate a higher quantity of grain. As a result, more harmful acid-resistant strains of E. coli, like the infamous O157:H7, evolved to survive. This is the dangerous strain that has found its way into our water, produce, and meat in recent years.

Michael Pollan, author of The Omnivore's Dilemma and In Defense of Food explains:

Most of the microbes that reside in the gut of a cow and find their way into our food get killed off by the strong acids in our stomachs, since they evolved to live in the neutral pH environment of the rumen. But the rumen of a corn-fed feedlot steer is nearly as acidic as our own stomachs, and in this new, man-made environment new acid resistant strains of E. coli, of which O157:H7 is one, have evolved. [The Omnivore's Dilemma]

2. MRSA

Methicillin-resistant Styphylococcus aureus (MRSA) is another bacteria that, thanks in part to factory farming, is popping up more than ever before. MRSA can be spread by human or animal carriers. It is abundant in our environment and its resistance to antibiotics can make it difficult to treat. More deaths in the U.S. are now attributed to MRSA infections than HIV/AIDS, according to a 2007 report published by the CDC.

No real research has been conducted on the presence of MRSA on animals in the U.S., but European studies show a "strong causal link" between MRSA and factory pig farms. Agriculture scholar Laura Sayre notes that "the bacterium is widely present on pig farms in Canada, which sells millions of live pigs to the United States annually, so it seems pretty likely it's in U.S. pig factories, too."

3. Campylobacter and Salmonella

Campylobactor is a foodborne illness that can cause diarrhea, nausea, fever, and abdominal pain. The infectious intestinal disease afflicts over 1.3 million Americans every year. It is also becoming increasingly drug resistant, according to the CDC, growing to almost 25 percent drug resistant in 2011, from 13 percent in 1997.

The bug is usually found on poultry, and a 2010 study by Consumer Reports showed that 62 percent of chicken sold in supermarkets is contaminated with Campylobacter.

The same study also found that 14 percent of chickens were contaminated with Salmonella, a similar but rarer bacterium that's becoming increasingly antibiotic-resistant as well. The CDC estimates that Salmonella is responsible for 450 deaths each year. According to a CDC Threat Report published this year, 8 percent of non-typhoidal salmonella and a staggering 67 percent of salmonella typhi are antibiotic resistant.

Fortunately, unlike MRSA and E. coli, these microbes can usually be killed by proper cooking.

4. Mad cow

While much rarer than the infectious bacteria listed above, bovine spongiform encephalopathy (BSE) can also be attributed to modern farming practices. Mad cow disease first appeared in the 1980s as a result of offal, a mixture of the organs and entrails of butchered cattle, in feed. Farmers quickly learned that cannibalism can cause infectious neurodegenerative diseases in livestock.

As a 2003 report published by the World Health Organization explains:

BSE is clearly linked to the practice of recycling bovine carcasses to recover so-called "meat and bone meal" protein, and then feeding this protein back to other cattle. If cattle are not being fed protein derived from the carcasses of ruminants (cattle, sheep and goats), there is virtually no risk of BSE. [WHO]

5. Obesity

In June the American Medical Association officially recognized obesity as a disease. Unsurprisingly, the nation with the largest industrial food industry also has the highest rates of obesity and diabetes. Every day factory farms in the United States grow 500 more calories per person than they did 25 years ago.

The country's industrial corn farming plays a major role. Government subsidies make production of corn, corn syrup, and corn-based processed foods very attractive to farmers. They also support the aforementioned corn-based diets that can cause a myriad of health issues and increase the fat content of conventionally produced beef.

Many of the changes to our food system brought on by industrial agriculture in the past 20 years are disconcerting, but consumers are beginning to take notice. As Pollan puts it:

Between the obesity epidemic, food safety issues (like E. coli and mad cow disease), concern about animal welfare, and a growing recognition that the American way of eating is making us sick, people seem ready to take a good hard look, both at the system as a whole and, even more important, at their own approach to food. [Penguin.com]