Question: Recently, I read a book called "Swallowed Star". In the story, a virus changes human DNA enough for humans to have unbelievable superpowers and skills. Some even obtain the ability to have their own gravitational pull and not breathe in space.

I want to know if it is actually possible for genes to be able to cause such things, or is it just impossible theoretically speaking if there isn't enough matter?

PS. Sorry if this sounds like a stupid question. I don't have the knowledge of space and viruses to answer this myself. Please forgive me.

Answer: 
Viruses and pathogens can affect everything that is based on genetic expression or biochemistry: there are known examples of pathogens changing the behavior of their hosts, for example, not forgetting that any pathogen tries to turn its host into a replicator of themselves.

What a virus cannot do is changing the laws of physics.

> 
> Some even obtain the ability to have their own gravitational pull and not breathe in space
> 
> 
> 

Changing the gravitational pull would require at least one of the three:

1. changing the gravitational constant G
2. changing the mass of the infected
3. changing the distance between the infected and the pulled object

1 is impossible based on what we know today about physics.

2 is trivial, a virus can induce mass weight or mass loss, but the effect is hardly appreciable, unless you want to step in the realm of mama jokes.

3 is also trivial and again hardly appreciable. We get closer or farther to many objects in our daily life, without even realizing that our gravitational pull on them is changing, because it's such a low pull that it can be taken as 0
I would argue that a sufficiently complex (that is, likely not natural) retro-virus could make some changes like super-strong, super-fast, likely significantly improved senses of sight and hearing. Even there it might only express (either fully or even partially) in children rather than already-grown individuals. And to effect children the retro-virus would need to infect gamete-producing tissues.

To do away with the need to breathe you would need to integrate an entirely different metabolic pathway, I suppose technically feasible, somehow, but seems unlikely. Even more unlikely for that alternative to be as energetically-favorable as using free oxygen from an atmosphere (that is, just as jet engines have better performance than rockets because they don't have to carry their oxidizer along with them). However, none of that seems as unlikely as modifying gravity.

None of those things are 'magical'. Basically imagine the most optimized organism you can think of along any given axis and it could probably be accomplished.