Prions were so named by Stanley Prusiner, M.D., a professor of neurology at the University of California, San Francisco School of Medicine, won the Nobel Prize for this discovery in 1997. Prions were found to be responsible for the disease Bovine Spongiform Encephalopathy (BSE), popularly known as Mad Cow disease. This disease was found to affect humans who eat the meat of cattle affected by BSE. In humans, the disease is known as Creutzfeldt-Jakob disease, It is a degenerative neurological disorder and is fatal. "The first symptom of CJD is rapidly progressive dementia, leading to memory loss, personality changes and hallucinations. This is accompanied by physical problems such as speech impairment, jerky movements (myoclonus), balance and coordination dysfunction (ataxia), changes in gait, rigid posture, and seizures. The duration of the disease varies greatly, but sporadic (non-inherited) CJD can be fatal within months or even weeks", says Wikipedia.
Now prions are just molecules of protein, nothing more. Prions are not cells, have not DNA, and cannot reproduce. This was the reason for the astonishment when prions were found to cause a disease. How can prions cause a disease? It was found that they really do not reproduce themselves in the cells they infect. Prions become problematic because they are not in "proper shape". Proteins, you see, are large molecules and they normally exist in a folded form. The way they are folded is very important for their function. If they are folded differently or get opened out, they do not function in the normal manner they are expected to. Some proteins open out, or denature, when they are heated or cooled beyond the temperatures they are normally expected to encounter. Apparently, this is the reason for egg white becoming, well, white when heated.
Now, what happens with prions is that they induce other protein molecules to fold in the way they (prions) are folded. That is, though prions cannot reproduce themselves, they get other protein molecules to become prions. Thus, when prions infect a cell, the population of prions in these cells increases. And those molecules of proteins lose the ability to function properly and contribute to more protein molecules getting converted to prions. This goes on till the infected animal dies. The new discovery is that prions also undergo evolution. "The study from Scripps Research Institute in Jupiter, Fla. found that prions can develop many mutations. Mutations that help the prions to withstand threats then tend to persist in a population of prions, while other prions are destroyed. This eventually leads the prions to develop adaptations such as drug resistance.", reports World Science. In other words, prions seem to show all signs of a living organism, including the capability to evolve according to changing environment.
Once upon a time, people believed that life could be formed from inanimate matter. Later we realised that inanimate matter cannot lead to the formation of life, that it was tiny, invisible, seeds or eggs that led to apparent growth of life forms from "dead" matter. But then, the theory of evolution came along and we explored the possibility of life having evolved from non-biological matter on the Earth. Then came Fred Hoyle and Chandra Wickramasinghe who declared that life originated in space and the Earth was seeded from space, and even that it may be life that is controlling the Universe. And, of course, in the 1960s, James Lovelock put forward the Gaia hypothesis where he defined Gaia as "a complex entity involving the Earth's biosphere, atmosphere, and soil; the totality constituting a feedback or cybernetic system which seeks an optimal physical and chemical environment for life on this planet." So was the whole system acting like a living organism?
Long ago, people thought of plants as non-living things. When bacteria were discovered, they were thought to be the smalles living organisms. Then came viruses, and they did not have a complete cell, only the genetic content, the DNA, or the RNA. Now how could that be a form of life? Should viruses be counted as life forms? It is essentially just a molecule, a molecule of DNA, though. But still just a molecule. How can it be a form of life?
And now here comes a simple molecule of plain protein (Imagine, just a molecule!) that appears to show signs of life. It not only creates more of its own kind, but also apparently undergoes evolution. Now, where is the "life" part here? What is it that gives it "life"? What really is life? Is the Earth living? Of course, we don't see any signs of life. The Earth doesn't reproduce itself as far as we know. We know so little about the Universe that I wouldn't dare to say that we know that planets do not produce more planets. Yes, this may sound totally absurd, but remember, I am only saying that we don't know. Once upon a time we hardly even imagined that a molecule could cause diseases, let alone reproduce and evolve. Once upon a time we believed that life on Earth is unique and that life formed on Earth. No longer. There is solid evidence to show that much of what we see as interstellar clouds could be microscopic forms of life. Once upon a time we believed that the Universe is expanding at an increasingly slower rate, and that it would stop expanding and start collapsing one day. No longer. We now understand (I wouldn't say know) that the expansion of the Universe is becoming faster, and we are not able to explain why.
In short, so long as we don't know what exactly is happening, we have to assume that anything is possible, however absurd it may sound, so long as we know that it is impossible. I know that the above statement is also ambiguous. What do we mean, for instance, when we say know? What we know today could turn out to be wrong tomorrow. Of course, that is how knowledge progressively approaches "truth". We cold go on wondering about such things, but let me stop before it becomes too metaphysical. My intention was just to point out that we hardly understand "life" and when we say that something is lifeless, we may soon have to change our opinion. Let us, at least, remember that we know so little about the world we live in.