Every time an organism reproduces, its DNA changes a little bit; this is called a mutation. Sometimes mutations are productive—they can make us run faster or process food more efficiently—and other times they can cause life-threatening genetic diseases like cystic fibrosis.
For organisms that reproduce very rapidly, these changes can be profound. Bacteria and viruses can replicate in under an hour, making mutations commonplace. Of the many mutations in a population, there’s almost always at least one that makes the population better at surviving it's current conditions.
If you have a bacterial infection and you treat it with antibiotics, it’s very possible that one of the mutations in that population could allow the bacterium to survive the antibiotic treatment. Because bacteria replicate so quickly, the population can become completely immune to the antibiotic treatment in just a few days, making the bacteria resistant.
The same is true for viral and fungal infections. If you catch a resistant infection, medicine doesn’t have a backup plan. Therefore we need to provide countermeasures, therapeutics, and diagnostics in real time to counter these evolving pathogens.