Your question is an extremely valid one. While it is true, that viruses are always changing, I think we should first address how viruses change.

We know of many human diseases that are caused by viruses like AIDS, chicken pox , colds, leukemia ,liver cancer, measles, polio (Believe me there are a lot more)

It is the distinguishing features of viruses that actually make them different. They include:
type of cell they can infect ,
host range (how many hosts, man vs. pig, etc),
virion morphology ,
genetic material ,
genome structure ,
genome sequence

Viruses can also have different morphology (helical, polyhedral, enveloped, etc.),and the Viruses "genetic material" can be either DNA or RNA.

This genetic material can change such that the proteins that it encodes (proteins that work to invade the host cell or take over its transcriptional or translational machinery) can be changed too, thus making a new viral strain. When this happens, the therapeutic agent doesn't work against that virus to deplete its populations within cells. However, if you look at the list of diseases caused by viruses, and you think about the history of humans, in general, viruses have become much less of a threat due to safer handling of food, loss of co-habitant (with human) rodent populations, and safer medical practices. Yes, it is hard to make a drug against an organism that is changing its proteins, but in fact, the harder part is making sure that the use of medication is REGULATED by overseeing agencies such that drugs that cure do not also have secondary (sometimes more harmful) effects.

Flu shots are a great cure for the common flu caused by the virus influenza. However, there are other drugs that also help with lowering the infection time, like Antiviral drugs can be used to help prevent and treat influenza. Some of these drugs are very powerful and more effective than vaccination. Of course they have often side effects and have to be taken right away when the infection starts.

So, as you can see, scientists are really very successful in many therapeutic advances against viruses. A good website to visit is the Center for Disease Prevention and Control:
CDC .

These sites have a lot of information on viruses and world health.
Cheers to your health!