For your first question, whether viruses are alive, viruses are not generally considered to be alive, although a lot of debate surrounds this question. If you look at a tree that shows all living organisms on it, you will not find viruses there. This is because viruses are completely dependent on another organism to live, and cannot survive without the other organism. Viruses cannot reproduce, or make more viruses, unless they are in, or on, another organism. But once they have access to the resources they need, viruses seem very much alive, as they reproduce and become part of the food chain. However, even though viruses are not generally considered "alive," some people think that they may be the origin to life on earth. Basically, viruses are complicated! I actually wrote an article a while ago on viruses, the debate over whether they are "living," and how they may be the origins of life on earth -- here is a link to the article if you are interested:

As for prions, prions are not at all considered alive. To understand why, you need to know what proteins are. Proteins are the functional units in your body -- there are many, many proteins in every cell in your body. Proteins are basically what makes up your cells, and so they are what most of your body is made up of. Different proteins have different jobs. There are proteins in your liver to make sure the liver as a whole functions correctly, and there are proteins that help make you have strong muscles. Every one of these proteins needs to have the right three-dimensional shape in order to do its job -- think about how you need the right shaped key to open a certain lock. Prions are proteins, but they are proteins that are not behaving properly. This is because their normal three-dimensional shape has changed, and they can no longer do their job. But not only can they not do their job, but they actual cause damage to the cells. But what is surprising to researchers about prions is that prions are infectious -- they can make more proteins have the wrong three-dimensional shape, spreading the damage they do. Usually a protein that has the wrong shape is not infectious -- usually the body can easily throw the misshaped protein into the trash, but this does not work with prions. You can learn more about prions from Wikipedia here:

That's a good question.Scientists have a lot of definitions for what exactly it means to be alive. In fact, I'm writing a little article about that question this month. A clever scientist found 123 definitions for 'Life' and compared them. He found out that most of them said something about Reproduction and Evolution or Variation, so he says "Life is Self-Reproduction with Variations." He doesn't talk about prions or viruses, but they are not alive, by his definition. Viruses need help from cells to reproduce, and prions don't exactly reproduce. They are proteins that are folded badly, and they make other proteins fold badly too.

Scientists are looking for life in outer space, on Mars and other places; and they are trying to figure out what to look for, because tiny living things might be very different from the living things we have on earth.

The textbook definition of life says that live needs to have all of the cellular machinery necessary for its own replication, which neither viruses nor prions have. However, viruses and prions, especially viruses, have a number of life-like attributes: they can reproduce, and viruses in particular evolve in response to natural selection. For this reason, I would suggest that excluding viruses from being alive is something of an artificial distinction.