Parasite that takes over cognitive functions I am testing an idea for a book. The backbone of the story is that there exists a parasite that can enter the brain of a creature and take over its cognitive functions. I imagine that it could wire itself in some central part of the brain or neural system, but I am not an educated physician. Let us assume that the being infested is a human. I would like some input:

1) Is this at all possible? I know it is possible with parasites that infest insects.

2) What area of the brain/neural system would it have to take over, or how at all could this happen?

3) The parasite just hijacks the brain, does not destroy it. If it decided to retreat, how much of the information/memories acquired during the infestation period could it carry with it? How much of the events happened during this period would be accessible to the subject after retreat?

4) If we assume a parasite that itself has some cognitive ability, and does not act just out of infestation instinct, how large would it have to be in order to be able to have its own (even basic) thoughts, and retain memories of the subject?

I would appreciate any input on these questions, and any related thoughts you might have.

This question has been put on hold because answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. With all due respect to the hard work of the moderators, I have to disagree. To begin with, all answers to all questions and even the questions themselves are exclusively opinion-based. Otherwise we would have to assume that there really is any truth that can be objectively be known and agreed upon. No one has ever been able to prove that anything besides our consciousness exits. But if even we have to tone down our agnostic perspective (why do we have to do that?) if you mean that there is not (in the world that you believe you perceive) enough evidence and research to support informed answers, then again this is not right. A user replied with an example of something like this occurring (evidence) and another with a very elaborate answer with examples and cited references to how something like this could happen. I strongly believe that more informed answers can come through.
I can't give you an informed response to questions 1 and 2 though I do know that parasitic wasps inject toxin into a spider, lay an egg on the back of the spider and cause it to spin a web that will protect the egg.  So some form of behavioural  control of spiders is possible.  Spider brains are much simpler than human brains but there was a paper by Strausfeld and Herth in Science April 2013 that argued that there are "deep similarities in how the brain regulates behavior" in both insects and humans.  There are similarities of function and structure between the 'central complex' of insects and the basal ganglia of humans. So I am guessing your parasite would need to target the basal ganglia.

However, the idea that the parasite might acquire memories and information is a totally different scenario and extremely unlikely.  Memory is a complex phenomenon but it appears to be largely based on the re-activation of particular neuronal pathways. Most memory is largely inactive most of the time, it has to be re-created each time (and hence is inherently slightly different each time).  I suggest the meaning of such activation patterns is intrinsically linked to a)  the other neural activation patterns occurring in the brain at the same time and b) the human body and its experiences. In any case, for a parasite to 'capture' memories it would need to be able to emulate those neuronal pathways; and even if it did, it would probably 'mean' something entirely different.  Even if it could 'copy' memories, it would not somehow take the memories away from someone.  Memories can be destroyed, or made inaccessible but only by disrupting the activation patterns (destroying neurons, blocking or destroying triggering mechanisms etc.). 

You also should be aware that even when neuronal pathways are activated,it does not mean that we are aware of them.  In fact we are unaware of most activated 'memory' (see Kahneman 2011).   There is a difference between implicit memory (and automatic behaviors)  and explicit memory.  Part of the brain, the left ventrolateral prefrontal cortex, is activated when we are consciously thinking or ruminating  about something, solving difficult problems, etc.  It appears to tap into memories and make them explicit so we can think about them or use them for computations etc.   As an aside, the LVLPFC is a long way anatomically from the basal ganglia.

Another important distinction for your thesis is the difference between procedural memory (memory of motor and cognitive skills etc.) that, for example,  allows one to type without looking at the keys: and declarative memory or memory about things.  The parasite I mentioned earlier presumably is able to trigger some part of procedural memory: but it seems highly unlikely that it might also trigger declarative memory, or even access declarative memory, which largely occurs in a different part of the brain.    However, declarative memory is severely affected by damage to the hippocampus so it is perfectly possible that a parasite might be able to induce amnesia.  However, if my very weak memory serves me, the hippocampus is not adjacent to the basal ganglia so it is unlikely that the same parasite could control behaviours and take memories.