@behaviourism, that is another good example. While antibiotics, immunizations, and other preventative measures seem like a good idea, they can backfire.
Antibiotics are only good against bacterial infections and diseases; against anything else, they are useless. Additionally, overuse of them can cause the same antibiotic to not help you the next time you have the same illness. Antibiotics also cannot be used preventatively unless in high quantities, as they are in the meat industry.
Immunizations, while different, have some of the same problems. One immunization can make your actual immune system weaken for a a few days, enough time to allow another, different strain of a virus to enter and cause problems; this especially happens to people who get flu shots when they are not in a high-risk group. For people of high risk, it is recommended despite the immune system weakness.