The spread occurs either by the passage of some DNA from the genetic material of the donor bacterium or by means of a plasmid , which is a sort of bacterial virus. When bacteria develop resistance to an antibiotic, they will survive exposure to the antibiotic. For example, if a patient requires a 10-day course of antibiotics to eliminate an infection, but doesn't finish taking all of the medication because the patient feels better, only the most sensitive organisms will have been killed and the more resistant ones will persist and flourish. The same effect is obtained by giving too low a dose of the antibiotic. Similarly, the prophylactic use of antibiotics, especially long term, invariably results in colonization by resistant organisms because sensitive organisms have been eliminated.
Infectious diseases remain a challenge.
It was thought that after World War II, epidemics could be banished by the use of antibiotics, vaccines, sanitation, and pesticides. In addition, practices such as refrigeration, sterilization, and disinfection have also decreased exposure to microbial pathogens. However, new diseases have emerged and old ones have reappeared. These infections can now circle the world in hours. The hope is that continuing research by pharmaceutical companies such as Bayer will provide new measures to treat these diseases.