The leading causes of staph infection are the unique survival tactics of the bacteria as well as antibiotic resistance produced by humans and include:
Unnecessary antibiotic use. For years, antibiotics have been prescribed for colds, flus, and other viral infections that don't require antibiotics and would normally clear up on their own. As a result, bacteria have grown immune to antibiotics because of decades of excessive and unnecessary use.
Antibiotics in food and water. In the United States, antibiotics can be found in beef cattle, pigs, and chicken. Runoff from animal feedlots finds its way into municipal water systems when they contaminate streams and groundwater.
Germ mutation. Sometimes antibiotics don't destroy every germ they target, so germs that survive treatment with one antibiotic soon learn to resist others. Bacteria mutate more quickly than new drugs can be produced and is the reason some germs end up resistant to just about everything, which is why only a few drugs are now effective against most forms of staph infection.