Recently in the UK, doctors saved a teenager with an antibiotic-resistant infection from death by injecting him with a set of specially trained bacteriophages. These are special viruses that have been selected and then "tuned" using genetic engineering techniques to attack specific bacteria. By the principle of action, they resemble professional hunter-rangers who track down their target in the human body and destroy it without affecting other tissues.
The creation of specialized bacteriophages is a fairly young field of bioengineering, but the basic methods of application have already been worked out. Scientists know where to find the right viruses, and then how to "train" them on the target bacteria, producing effective strains. There are bacteriophages of a wide profile, if highly specialized, and in some cases they use a "cocktail" of several strains at once. Before the introduction, samples are taken from the patient to check the quality of the viruses in the laboratory, after which a course of treatment is prescribed.
Another popular technology is the use of modified viruses that "train" the human body to develop the correct response to real pathogens. For example, the GBV-C virus extends the life expectancy of HIV patients. Once in the body of carriers, GBV-C blocks receptors that are involved in the penetration of HIV into cells, and provokes the activity of interferons and cytokines. In a word, it creates "interference", physically preventing harmful viruses from reaching the target, simultaneously stimulating the body to fight.
There are also examples of the use of noroviruses to protect the intestinal environment of experimental mice with destroyed natural microflora. With the help of genetic engineering, scientists have learned to endow harmless viruses with characteristic signs of dangerous ones - such "trainers" travel through the body, irritate the immune system and provoke the production of antigens. This method is similar to all known vaccinations, but safer than them - and also has great potential for "training" the immune system to respond to any arbitrary threat.