Viruses and bacteria are two separate classes of organisms. Viruses do not have the capacity to reproduce on their own; they must have a host to do so. In that, they are in the strange category of being sort-of alive, but not fulfilling all the requirements to be truly considered alive (which includes procreation). Bacteria, on the other hand, are single-celled organisms that meet all the requirements of being alive, and hence can procreate on their own through asexual reproduction (mitosis).
Why you are having difficulty understanding evolution is that your definition is incorrect. Evolution is not about survival of the fittest. It is change in genetic frequencies in a population over time. Evolution only can occur in a population, and only over time/generations. It is based on the following processes, working together:
1. Mutation- this is a random process that happens when DNA replicates before mitosis (cell division) or meiosis (the process that yields the sex cells of sperm and egg). Essentially, all life on earth is made of cells, and when these cells divide, they must make copies of their DNA for the new cell. When this occurs, the process sometimes randomly gets screwed up and mistakes are made in the copy. The new cell thus has a slightly different blueprint. Sometimes this change yields a change in the outward form (prototype) of the cell, and this change can be beneficial to the organism, neutral, or negative. If it is beneficial, it will cause a change that is advantageous to the organism's survival and reproductive capacity in its current environment. If neutral, it will make no effect at all. If negative, it will reduce the likelihood of survival and reproduction. In single-celled organisms, like bacteria, they only reproduce asexually but they do so at a very rapid rate, so the mutation rate is correspondingly high compared to more complex organisms. This is why we're constantly needing new flu vaccines and antibiotics- the random mutations of bacteria eventually produce strains that are resistant to the ones we have. In more complex organisms, like ourselves, we have the added process of sexual reproduction, which combines material from mom and dad, and so in those cases, only mutations in the sex cells matter for evolution, because only they will be passed on to the next generation.
2. Natural Selection- this is probably what you are referring to by "survival of the fittest."
Evolution really isn't about survival of the fittest. It's about who reproduces the most. This can be accomplished by living longer, as long as one's reproductive lifespan also increases, but it can also be accomplished by other advantages- like being able to protect one's mates better, or attract more mates, etc.
Natural selection is the process by which traits in the organism interact with the environment. Different environments can cause the same trait to be neutral, good, or bad for the organism. Traits that are bad for the organism will cause it to reproduce less, thereby passing on fewer of its traits to the next generation. In time, bad traits will either disappear completely from the gene pool (in extreme cases), or will become very rare. Traits that are good for the organism, on the other hand, will cause it to reproduce more, thereby passing the trait on in higher numbers to the next generation. In time, many or all individuals will show the trait.
A great example of how this works in humans is skin color. Around the equator (Africa, Central America, parts of India and Asia), the sun is very fierce. It is a good trait to have dark skin, because it blocks the ultraviolet light and keeps people from getting skin cancers, heat stroke, etc. So it is the most common trait, although albino and lighter skinned and eyed people occasionally crop up (there's an explanation for that, but it's long and detailed, so I'll leave it out). In the northern areas (even more so than the southern areas, due the tilt of the earth's axis), however, the dark skin trait is not good. It blocks the absorption of the sun's rays, and this is how we produce vitamin D, which is needed to avoid diseases like rickets. Now, along the equator, there is so much sunlight that it's OK that not much is absorbed, because it is plentiful enough to ensure that the bit that is absorbed is more than enough to produce adequate vitamin D. But in the far northern areas (think Scandinavia and the UK, for example), there is very little sunlight, and nearly none during half the year. Thus, over time, the population that migrated up there favors the trait more and more of light skin, which absorbs more of the sunlight. So you see, the same trait can be good or bad depending on the environment. If you're in Africa, and you are born with light skin, you'll have less kids because you'll be plagued with melanomas and heat stroke. If you're in Norway, and you are born with dark skin, you'll have less kids (and therefore pass on your genes less) because you'll be plagued with rickets.
3. Gene Flow- this is the process by which populations interbreed, thus making their genetics more similar to one another. Less species divergence is possible the more populations are interbreeding, because they are sharing mutations that arise over a broader population. Humans are fabulous at gene flow, which is why we're one of the most wide-spread species on earth, and yet still are all the same species.
4. Genetic Drift- this is the process by which populations become isolated, thus reducing interbreeding and causing their genetics to become more dissimilar. More species divergence is possible, because no one is sharing their genes. This is especially common due to geographic barriers, though that is not the only cause. Islands are a famous case. When organisms get stuck on islands and can't interbreed with those on the mainland, their population goes through a "bottleneck effect," in which only a few members are present that do not adequately represent the parent/mainland population. As these few members interbreed, their genetics become rapidly divergent from the parent population. You can often see this in only a few generations. This sets the stage, if sufficient time passes, for species divergence.
You see, evolution is a random process, as it is based on mutation, which occurs more or less randomly (there are a few cases, like radiation, that causes an increase in the rate of mutation). There is no "will toward complexity" or anything of the sort. Mutation just causes new traits to crop up occasionally, and these interact with the environment through natural selection, gene flow, and genetic drift, sometimes resulting in evolution and sometimes not. Over time, as the number of new traits increases in a population, it can eventually become so different from the original/parent population that interbreeding is no longer possible. At that point, a new species is born. The amount of different genetic material necessary for some major differences is not a lot- chimps and us humans share 98% of our genetic code. But that 2% makes a big difference.
Hope that helps explain it a bit.
