Antibiotic resistance is the ability of a microorganism to withstand the effects of an antibiotic. This resistance develops through gene action or plasmid exchange between bacteria of the same species. If a bacterium carries several resistant genes, it is called multiresistant or, as it is often described, a 'superbug'.

Essentially, antibiotic resistance develops as a result of natural selection. The antibiotic action is an environmental pressure, and those bacteria with mutation allowing them to survive will live on to reproduce. They will then pass this trait to their offspring, which will be a fully resistant generation.

Several studies have demonstrated that patterns of antibiotic usage can have a dramatic affect on the prevalence of resistant organisms. Other factors contributing to resistance include incorrect diagnosis, unnecessary prescriptions, improper use of antibiotics by patients, and the use of antibiotics as livestock food additives for growth promotion.

Staphylococcus aureus (Staph aureus) is one of the major resistant pathogens. It is found on the mucous membranes and the skin of around a third of the population it is extremely adaptable to antibiotic pressure. It was the first bacterium found to be resistant to penicillin; it was discoverd just 4 years after penicillin began to be mass-produced.

Penicillin-resistant pneumonia (or pneumococcus, caused by Streptococcus pneumoniae) was first detected in 1967 as was penicillin-resistant gonorrhea. Other strains with some levels of antibiotic resistance include Salmonella, Campylobacteria, and Streptococci.

Unlike antibiotics, vaccines do not contribute to resistance. Vaccines work by enhancing the body's natural defenses, whereas antibiotics operate in lieu of the body's normal defenses.