If a single drug is given for the treatment of tuberculosis, there is increased chance of development of resistance to the drug. The incidence of resistant strain in an individual will be about 1 in 10^6 to 10^8. But an indiviual be infected with 10^10 to 10^12 organisms. So if a single drug is given, the number of remaining organisms will be too high for the immune system to remove them. The probability of an organizm resistance to the drug is independant of the probability to develop resistance to other drugs. Hence when two drugs are combined, the probability of resistance will be about 1 in 10^12 or less. The remaining organisms can be easily removed by the immune systems. This is the rationale for multi-drug therapy in tuberculosis.
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