If microbes become immune against the drugs used to treat the infection, it might take longer for you to recover, and you might die. For example, some antibiotics might not be able to treat bacterial infections anymore. Therefore, other antibiotics may need to be used, which may have more side-effects. And using other antibiotics will result in bacteria becoming immune for these drugs as well...
 
Eventually, we might end up in a situation where no more drugs will be available to treat infections.