Answer:
Infectious dose refers to the minimum number or amount of pathogens required in order to produce a disease or its symptoms. In the given case, the bacterium is non-resistant and is treated with antibiotics.
The antibiotics help in neutralizing the infectious agents; however, full course should be done to eliminate the infectious agent completely out of the body.
If the course is left in between then, there are very high chances that some bacteria may survive. They might not produce disease or symptoms as they would be less than infectious dose.
However, with time, the survived bacteria gain more and more mutations during replication which can develop antibiotic resistance capability in them.
In addition, it is also possible that the survived bacteria can get R-plasmid (which has antibiotic resistant gene) from body's normal flora or from any other infectious agent present in the body by the process of transformation or conjugation.
The R-plasmid enables them to survive even in the presence of antibiotics due to which the treatment of the disease might become difficult.
