For each seperate infection, doctors have one or more standard first-line STD antibiotics treatments, based on best practice and established medical experience. These treatments evolve over time according to the epidemiological situation of infectious diseases. For example, half of pneumococci (responsible for respiratory infections) are known to resist penicillin, so doctors tend to choose another drug from the same class or an alternative antibiotic. The exact choice of antibiotic treatment also depends on the affected organ: some substances tend to concentrate in one or more organs and are therefore a recommended treatment when infection occurs. Finally, the choice also takes account of the patient's particularities: if the patient has already shown an allergic reaction to penicillin, for example, the entire family of beta-lactams is prioritized.Sometimes an STD antibiotic treatment fails, although it has been properly prescribed and taken by the patient. These failures are related to antimicrobial resistance and the physician must then modify his prescription: this is called \"second-line\" antibiotics. These are drugs more rarely used, intended for these cases of resistance. Some are even reserved for hospital use, to overcome the repeated failures of more common
antibiotics and treat multidrug-resistant nosocomial infections. To choose the new treatment, the doctor may ask for an antibiogram.The spectrum of an antibiotic is the set of species of bacteria that are naturally susceptible to this antibiotic.Sensitivity expresses the effects of various antibiotics on a particular bacterium. To measure this sensitivity, a special examination is performed.