Modern commercial farming uses a lot of antibiotics, and, as a consequence, we’re beginning to see them breeding drug resistant bacteria (see here for exponential growth demo). Jeremy Laurance reports on one bug (MRSA ST398) now being found in milk.
Three classes of antibiotics rated as “critically important to human medicine” by the World Health Organisation – cephalosporins, fluoroquinolones and macrolides – have increased in use in the animal population by eightfold in the last decade.
The more antibiotics are used, the greater the likelihood that antibiotic-resistant bacteria, such as MRSA, will evolve.
…
The MRSA superbug can cause serious infections in humans which are difficult to treat, require stronger antibiotics, and take longer to resolve. Human cases of infection with the new strain have been found in Scotland and northern England
— Laurance (2012): New MRSA superbug strain found in UK milk supply in The Independent.
Note that consumers of milk don’t have to worry because the milk is pasteurized.