While a lot of people died during the spanish flu pandemic, many people were also infected and survived. thinking like a virus, if viruses could think, why is it beneficial to make a human very sick but not actually kill the human? '

Viruses have three goals when they infect the body: mutation, multiplication, and transmission -- grow stronger, make more, spread more.

In order to mutate, the virus must infect a host and combat the host's immune response. Making the host as sick as it can, allows it to encounter the best that the host's immune system has to offer (e.g. the immune system increases temperature to create unideal conditions for invading pathogens). The virus, in turn, encodes itself with the instructions on how to adapt to the markers and attacking cells that accompany the host's immune response. Thus, it mutates into a stronger and or more resilient version of itself.

However, if the virus kills the host, it may not being transmissible to other potential hosts. If the host dies, the virus itself will soon die too, as the environment is no longer suitable for mutation and multiplication. Deceased people do not mobilize to where other potential hosts congregate the way the living love to gather in crowded and or shared spaces.

Thus, it would be beneficial for the virus to make the person sick enough to mutate into a strong version, but also not too strong so that it can lay latent and multiply, such that when the host believes they are no longer sick, they can transmit the virus to the next host and repeat the cycle.

The many strains of SARS-CoV-2 can serve as an example of this concept. Many did not believe they were contagious after bouts with the coronavirus and spread it to the next person.

In short, it would beneficial for the virus to make a human sick but not kill them because it would provide the virus the opportunity to mutate and multiply if they were kept alive.