Final answer:
Excessive use of antibiotics results in antibiotic resistance, leading to more serious infections that are difficult to treat, and has become a public health crisis.
Step-by-step explanation:
Excessive antibiotic use has become a major global problem because it leads to antibiotic resistance. This occurs through natural selection; resistant bacteria survive antibiotic treatment and reproduce, leading to a higher proportion of resistant bacteria.
Such bacteria can then spread among humans and animals, which complicates the treatment of diseases. In livestock, antibiotics are often used in low doses to promote growth, which maximizes the chance of resistance developing. This resistance can be transferred to humans, exacerbating the issue. Overall, antibiotic resistance contributes to an increase in serious infections that are difficult to treat and has become a public health crisis.