Re:
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Yes...however, not all parasites show up in the bloodstream; therefore, using a scope makes it somewhat moot.
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True, IF all one observes is the blood.
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At that point, a guess on the situation would certainly be in order.
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Maybe, or maybe trying to look at another type of fluid for clues. There's one thing I can say about pathogens in general: they're always leaving their trash behind (toxins), so here's where what I'm calling 'The Art of Microscopy' REALLY comes in, when it comes to gathering those 'clues'. In situations like these and with tools such as those, I don't think that to guess is really that necessary, but of course that's an opinion.