What are the advantages and disadvantages of being tested for diseases?

There are a wide variety of diseases that you could be referring to in your question. I will try to address them in two groups: infectious diseases and genetic disorders. 

In the case of infectious diseases, the main advantage of getting tested would be the disease could be identified and proper treatment could begin. For example, if you have strep throat and it can be diagnosed quickly you can begin the correct antibiotics right away and avoid days of feeling miserable. In a more serious example, if someone gets tested for HIV and it is positive, they can begin treatment quickly which can improve their quality of life going forward. The only disadvantage for getting tested for an infectious disease is that there is sometimes a social stigma tied to certain diseases and treatment can be expensive. 

In the case of genetic disorders, there are more issues to weigh. In the case of genetic disorders that can be passed on to children, a couple may want to get tested to understand the chances of passing on a disorder. For example, if someone has a family history of cystic fibrosis, they may want to get tested to know if they are a carrier that could pass the gene on to children. Some people choose to have genetic testing for disorders that can show up later in life. This is the case for the gene for Huntington's Disease. Huntington's Disease is a devastating neurodegenerative disease that usually shows up later in life and there is no cure. By being tested at a younger age, someone can find out if they will eventually experience symptoms of Huntington's Disease. This is a huge decision that will likely affect how that person lives the rest of their life. 

With genetic disorders, some advantages to testing include possibly being able to prevent passing certain disorders to children and better treatment for different disorders since they will be correctly diagnosed. Some disadvantages include the fact that the results of the test can be life changing and can lead to very expensive treatments. In the case of testing to predict future disorders, such as Huntington's Disease, one needs to decide how the results will affect them and what they may do differently in life depending on the outcome. It is a very personal decision and can be different for each person.