DNA mutations in humans are generally bad, but why in viruses make their bodies stronger?

Mutations of the genome are neither bad nor good. They alter. If the product of this alteration is advantagious to the virus or any organism, they may gain some advantage to reproduce better. If the alteration is deleterious, it may be bad for the virus and any organism.

With humans you see bad results with viruses not (they are too small).

The effects of mutations
As have been already pointed out, mutations are neither good nor bad - they are simply changes in the DNA sequence. The effects of mutations on humans and viruses are rather different, since humans are multicellular organisms - mutations in one cell do not affect the whole organism, but only this cell (except for the germline mutations in sperm and ovum, before these cells start to divide and become an organism with trillions of cells).

A mutation in a human cell may have different effects:

• if it occurs in a coding region of a protein, this protein may start working better or its functioning may be disrupted (the latter is more statistically probable), leading to cell death.
• if it occurs in a regulatory region, it may dirupt normal functioning of the cell, e.g., making it divide uncontrollably, and thus creating a cancer tumor.
• it may also happen in a unimportant region, conferring no special changes. Note also that severe disruptions, such as thsoe leading to cancer, are usually not a result of a single mutation, but an accumulation of many of them.

A mutation in virus may equally have different effects. Most mutations have either negative effect - producing a non-functional virus, or a neutral effect - resulting in no special changes (particularly the so-called synonymous mutations, which do not change the identity of the encoded amino-acid). Some mutations change the proteins in the virus capsid, resulting in viruses which either have different properties vis-à-vis antibodies or can better attach to cellular receptors - these are beneficial for virus survival.

The important difference with the hma, where mutations affect only one cell, is that a single infected cell produces multiple new viruses (from dozens to millions of them,d epending on the virus) - some of them will be disfunctional, some as good as their parents, and some more fit.

Classification of mutations
As could be already conjecture from the discussion above, depending on the effect of the mutation on the new viral particles, they can be classified in deleterious, neutral and beneficial.

Mutations and fitness
A single viral particle being more fit would not pose much problems. However this particle may infect another cell, producing new generation of fit particles, which will replicate again and so on. In other words, what makes a mutation good for a virus is the natural selection among the millions of viral particles.

On the other hand, when we speak about a mutatuion being bad for a huma, we speak about the effect of the mutation on a single individual. In otehr words, we are comparing here appels and oranges. If we were to make an equivalent comparison, we should consider a human population over the course of thousands or even millions of generations. We would see in this case mutations can have similarly different effects: the Neanderthal "human strain" turned out to be less fit and disappeared, whereas the homo sapiens "strain" has survived and populated the whole Earth.

References:

• Population genetics
• Viral ecology
• Principles of virology