Intensive agricultural operations, commonly referred to as factory farms or CAFOs (concentrated agricultural feeding operations) pose threats to human health because of the massive amounts of animal waste that they produce. Whereas the waste from smaller livestock herds can easily be cycled back into the soil, even acting as a fertilizer, large operations create such huge quantities of waste that become unmanageable.
According to the EPA, CAFOs in the US produce more than three times as much waste (urine and feces) as the entire human population of the US combined, however there is no regulation requiring CAFOs to treat waste. Typically, this raw sewage is flushed out of the farm buildings, pooled into massive manure lagoons, and sprayed onto nearby fields as fertilizer (though for most operations, the soil is unable to absorb the quantities of waste that are produced).
Manure lagoons create vapors that can cause respiratory problems for nearby residents and often lead to run-offs that contaminate drinking water and cause massive fish kills if they reach waterways. Furthermore, untreated animal waste has a very high concentration of pathogens that can be dangerous and even fatal to humans (Bird Flue, Swine Flu, and Mad Cow Disease all came out of CAFOs, and E. Coli, Salmonella, and other outbreaks have become more prevalent because of CAFOs.)
Food and Water Watch has created an interactive Factory Farm map that enables one to look at the distribution of factory farms across the US down to the state and county for a number of farm animal types. What concentration of CAFOs are there in your home state?