The body is a wonderful system, and fighting off infections is something that it does very well, normally. Over the years, however, some of the infections that we get over and over have figured out ways to change themselves so they can continue to infect us.
The use of antibiotics has given us a way to help our bodies fight infections, but also has caused changes in the way the bacteria shift to avoid them. Some of these bacteria have become so clever at hiding from the drugs we use that taking a dose of them will not kill even the majority of them.
How Do Infections Change?
Once a virus or bacteria is in our bodies, they want to spread. They do this by duplicating themselves over and over. As with any process, there may be errors in the way they are copied. Think of a printer when it is close to running out of ink: some of the letters may blur, or a section may be lighter than the original. This is similar to what happens with the virus and bacteria: changes happen. And some of those changes may be large enough that the drug used to treat the infection can't recognize the changed virus or bacteria as a threat, and don't try and kill it.
These, of course, then continue to breed. Other changes may happen: with some infections, the number of bacteria doubles every hour. So you still have the infection, it just now isn't being treated by the drug used to fight off the original version of the infection.
Once a bacteria or virus has changed this far, the original treatment is no longer going to help you. A different drug, sometimes in the same general 'type' as the original drug used, will be able to combat the infection. After a time, however, the virus may change enough to be resistant to this version, as well.
This is the reason there are so many antibiotics, antivirals, and antimicrobial medications available today. Some people, in the course of fighting off an infection, become sensitive to the drug, and an option is needed if they get infected again. A range of options have been developed, so we can continue to fight infections.
What Happens When An Infection Can't Be Fought Off?
Your professional will normally start you with a typical treatment, after looking at your health, history, and studies in your area to make sure that the infection hasn't changed enough for the usual drug to be useless.
If this does not work, depending on your condition, you may need a second course of a different drug, or possibly a stay in the hospital to give you a stronger dose of a different drug that has been proven to work for your condition.
What Can I Do To Help?
The best help that you can be is not to ask for drugs that you do not need. A cold cannot be helped by antibiotics, nor can the flu in most cases. This also will prevent your body from having a batch of drugs in it that may have helped fight off the infection, but instead is allowing those changed bacteria or viruses to grow, and maybe be passed on to another person.