Natural selection explains to doctors that viruses are perfectly capable of evolving in order to combat their medications. In actuality, a virus is not an "idiot" when it comes to treatment, but is a living entity that is able to survive in human beings longer than is commonly believed. Although viruses have been referred to as enemies of the human race, we haven't taken that term any further until fairly recently; viruses were thought of as either being harmful or dormant, and scientists could terminate the harmful ones with medication and tackle the dormant ones should they do damage to a human in the course of its spreading around the planet.
Just like human beings, viruses are alive and thriving. They reproduce, albeit at a much faster rate than us, spread from place to place and consume resources, and are almost infinitely varied. There is not just one strain of malaria that exists in this world, but thousands upon thousands that live in human bodies and multiply. Natural selection comes into play when medication is developed to combat the most prevalent and dangerous forms of the virus. The strain of the virus that the medication is designed to attack will be terminated by it, but the strains that can resist the medication will still linger in the infected body. Over time, which could be anywhere from a few days to a few weeks, the resistant strains will repopulate the host, and the medication that was previously used to fight the eliminated strain of the virus will prove to be ineffective.
If it's hard for you to visualize a virus coming in many different forms, some of them resistant to drugs and others not, then we can use the human race itself as an example of a population that has been naturally selected. During the 1350's, over a third of Europe's population was wiped out because of the Black Plague. Scores of humans died because their immune systems were too weak to fight the disease, but the other two thirds, the resistant ones, survived and repopulated the country with a variation (just like a "strain" of virus" of human beings that were all resistant to the Black Plague. As stated before, a human being isn't very different from a virus, and the force used to fight humans, the Black Plague, was ineffective once humans had been naturally selected and none were susceptible to the horrific disease.
Just like any other trait, a human's susceptibility to a virus is a genetic factor that is inherited, but can also be mutated at random. When so many human beings, an over-abundance, if you will, are created, the law of variation is still observed; there are bound to be "mistakes" when organisms are created. An example of such a mistake would be the creation of humans that could not fight the Black Plague. Of course, this mistake was eliminated once the plague killed everyone who was unfortunate to inherit it, so variation in organisms really works both ways. Mistakes and mutations are necessary to the process of evolution, and mistakes are created when too many individuals exist. Consistency in nature is exceedingly rare.
Adaptations are essential to any organism's successful evolution. Increased fitness is the result of a successful adaptation by an organism, because fitness is defined as an organism's tendency to survive and reproduce in its environment. Viruses adapt to their hosts simply by "slowly" changing as they evolve inside a body. By slowly, I mean however many hours it takes for a generation of the virus to pass. A virus that may initially seem to do little harm to a human, such as the earliest stages of the Black Plague, will multiply, spread, and grow more comfortable inside the infected human. They adapt to their environment in a similar manner to other organisms, so doctors have to constantly be on their toes when trying to quell an outbreak or epidemic.
Let's look back at the HIV/AIDS virus, as this is a prime example of the enemy that doctors have to face when trying to cure patients. According to researchers at the University of California-San Diego, the virus is protected because of its sheer variation. Although the virus can be temporarily tamed, stopping its progress has proven to be ineffective at best due to its versatility. Of course, the virus also enters the body by using an already opened door: malaria is rampant in locations such as Africa, and there is a link between the people who have been infected with malaria and people who catch AIDS. When someone catches malaria, his or her body produces proteins to protect itself, which kill off the malaria, but the HIV virus uses these proteins to enter the body and infect a human's cells. In a sense, the virus isn't self-sufficient until it enters the body, but it multiplies exponentially and destroys a human shortly after infection unless several treatments are started.
Scientists constantly need to develop new medications because new viruses are being developed. This can be compared to war; in reality, it is a war. Just as humans fight with each other and use advances in technology to their advantage, humans fight viruses and use technology. But just as in all wars, the other side has technology too. The virus is capable of countering our best efforts just because its different strains are naturally selected, so many doctors understand that a patient living with AIDS who seems to have positively responded to a medication doesn't have long before he or she needs to start a new round of a different medication.
Now, today's doctors understand that a virus is indeed an organism that has to live side-by-side with human beings, but during the time of the Black Plague, it was believed that this was God's way of punishing humanity for its crimes. Very little, if any, scientific or medical explanation was given for the plague, so the people really didn't have an idea of what they were up against. But today, doctors know that humans have an enemy that is almost perfectly explained by science; science answers one set of questions, such as "how" things will come to be in this world, whereas religion answers another set, the "why" behind all events. Scientists do not know why things happen, such as why bad things happen to good people, but they do know how bad things such as the Black Plague became involved in the lives of good people.
The theory of natural selection personifies the virus in the eyes of the doctors and teaches them that the virus will get rid of its own mistakes over the course of time. After all, because the word "virus" rarely leaps to mind when the word "organism" is mentioned, many people don't understand the concept of evolution in beings other than plants and animals. In the case of the AIDS virus, natural selection tells doctors that new medications are in high demand for a patient, but it also raises a question: Will the AIDS virus eventually evolve to the point at which it is immune to all modern medicine, or will scientists and doctors exploit its weaknesses and terminate it before it can evolve to that extent? No matter which outcome scientists predict, the question forces scientists to put more effort and research into the development of medications to "win the race" against this deadly virus, this enemy.