First of all it is important to note that, within certain limits, human DNA is not much different than, say, a mouse DNA: it has the same structure, it is constituted by the same bases etc etc. Therefore it is teorethically possible (leaving aside ethical issues, of course) to selectively modify it as you would do for a mouse. There are, however, some technical issues and obviously many ethical ones.
To answer specifically to your questions
Is it even theoretically possible to alter human genetics using only an injected substance?
Yes it is. Aside from the good example of radioactive substances given by Alexander, another good one (and probably one that is more common) is that of chemical mutagens.
It is important to understand that the mutation caused by these substances are random, or only partially selective.
For instance, certain substances, called intercalating agents can interpose themselves in between the bases of the DNA, interfering and possibly giving rise to mutations during DNA replication.
Other substances such as alkilating agents are a bit more specific, although still not gene- or locus- specific.
For instance ethylnitrosourea (ENU) favours A->T base transversions and AT->GC transitions.
I'd imagine trying to get a single DNA change to propogate through the entire body without being killed off by the immune system would be near impossible.
The immune system would not necessarily kill cells with mutations. Think of what happens during cancer developement.
Also, an extremely important point: mutating a somatic cell (i.e. not a sperm or an egg) can induce a mutant phenotype in the individual but it will not be transmitted to his/her progeny.
Since we are aware of genetic markers that are related to predispositions to certain medical conditions (e.g. heart problems, cancers, high blood pressure, strokes, etc), is it possible to modify human DNA to create these issues in a living person?
As Alexander says, there is much research in this field.
Some attempts of gene therapy have indeed been made, and some have a big potential. We are, however, still far away for gene therapy to become a routine treatment.
Also, remember that:
- not all diseases are genetic diseases
- not all genetic diseases are monogenic: some derive from a complex interplay of different factors and hence curing them is not as simple as providing the correct version of the mutated gene.
Has much research gone into how genetics affects thought and opinion, or the way we perceive and react to situations at an emotional level? If so, is it theoretically possible to alter this at a biological level?
Yes, but I would say that we are very very far away from a complete understanding of how thought and emotions are coded in our brain. The very difficult point is that one needs to study these events at multiple scales. We can study a big region of the brain and say that it is involved in fear coding, for instance. But then if you want to go deeper and study what happens at the single cell level you need to look at a lot of different cells to see how they interact between each other.
From the other side of the "size scale", it is possible to study biochemical processes in the single neuron, but then again it is difficult to study how these processes are integrated in big networks of cells, especially in vivo.