Human, in particular, are vulnerable to virus infection. what is the benefit of this devastation and why? 1. lowering human-population. 2. evolution. 3. increasing viral population. 4. none.

This is a very vague question. Beneficial to whom or what?
To earth/ecosystem? If we are talking about a system, then we need to define what is a good system. To me, a good system is a stable system, that is robust to perturbation. Diversity introduced by virus contributes to the robustness.
To human civilization, none most likely. I don't think lowering population is beneficial in the long run, so strike that out. Diversity is caused by mutation, while some virus can cause mutation, most do not. The selection process posed by virus actually decrease the diversity, as obviously only the fit ones are likely to survive and make offspring.  
TBH, this is kind of a dumb question. Cannot believe I wasted 5 minutes answering this.

La aparición de enfermedades emergentes por agentes virales es cíclica, y se requieren sistemas de vigilancia epidemiológica centinela específicos a determinados  virus para detectarlos oportunamente circulando en las sociedades. Asimismo, preparar vacunas creadas con tecnología reconbinante usando partículas de ARN. 

 Answer 4: NONE While viruses can cause illness and even death in humans, they don't offer any inherent benefit, and the statement framing it as "devastation" is inaccurate.

In case of a viral infection, as the infection spreads there could more cases mortality thereby lowering human-population. Over a period of time, humans evolve to adapt to this infection which forces the virus to further evolve. This evolution increases the viral population. Considering this hypothesis as a possible scenario, it could be fare enough to state that all the 3 options are true. 
However, during the breakout of a viral infection there may have been several industries that have seen a boost in its economy which is seldom discussed.     

Viral infections in human is vital for creating a "Selection pressure" in a way promoting evolution. It can very well be envisaged based on the presence of retroviral elements in the human genome. This in a way creates a more resilient population that can withstand the infection if it happens in future.  

Evolution because in many cases, virus infections can result in human diversity by either promoting divergence or killing population groups that are less resistant to infections. For example, in many viral attacks, indigenous populations suffer more than native individuals. This is worst when viruses carried by humans or other animals migrate to new populations, which can significantly contribute to population decline. 

Viral infection, even when it produces considerable morbidity, does not necessarily result in death or the lowering of population. The burden of the associated morbidity may even offset benefits from a lower population, depending on how you choose to measure these. 

Evolution driven by infection can result in populations that are less susceptible to infection. However, this is only a benefit because infectious agents are present in the first place. Who knows what apex of fitness can be achieved in other domains if infectious agents were not there to impose unrelated selection pressures? Viral sequences integrated into the human genome facilitate recombination, and this can be used to support an argument for more general evolutionary benefit. However, the attendant vulnerabilities caused by viral reactivation are not themselves beneficial.

If anyone truly thinks that increasing total virion copy numbers is an intrinsic moral good, then it would amuse me greatly to learn more about their system of morals. Ultimately, the 'devastation' caused by viral illnesses is a predicament that populations have to find their way around in order to persist rather than something intrinsically beneficial.

Lowering human population. Because there is no cure for viral infection. Eg. Millions of people died in developing countries because of HIV infection, millions of children lost their parents and forced to live in poverty, miserable life and death. Other recent devastating experience is Covid-19 infection. It is a cause for death of several millions people across in the world. In both cases there is no curative medicine. So what?......