Most people have been immunized against some diseases. This has lead to a decrease of infections with these diseases in the industry nations and worldwide. Naturally-occurring cases of smallpox have been totally eliminated. But even though the methods of vaccination have been much improved over the last decades, they still hold several disadvantages.
Some are based on damaged or destroyed pathogens. If the damage was not completely successful, the vaccination itself can cause the disease it was intended to prevent. Even a "safe" vaccine can cause severe side-effects. Also, you can only be immunized against a single or a few variants of a disease at the same time. Many of today's vaccines have to be constantly cooled, making transport and storage hard and expensive, especially in warm regions of the earth.
DNA vaccination might potentially solve these problems. Instead of taking a damaged pathogen, a single gene from that pathogen is artificially copied and multiplied. That gene is then injected into a muscle. For some unknown reason, muscle cells tend to take up this gene and use it as one of their own genes, making the product the gene describes. The immune system will recognize that product as foreign, and remember it, just like it does in the "classic" vaccination.
What is the point of this? First, the gene is made artificially, and can therefore be much more pure than any vaccine made directly from pathogens. Second, it is only one of the many genes necessary for the pathogen to reproduce, like a tire is for a car. That small part is enough for the immune system to recognize its enemy, but not enough to become a danger to the body. Third, several different genes can be mixed and injected simultanously, making it possible to vaccinate against many variants of a pathogen, or against several different pathogens, at the same time. Finally, the genes are cheap to produce, don't require cooling, and can be stored for years.
Today, DNA vaccination is still an experimental procedure. Some DNA vaccines have been tested on humans and proven to be harmless. However, the immunizing effect has not been shown to be of practical use.