We know [that HIV can't be transmitted by mosquitos](https://biology.stackexchange.com/questions/44868/can-hiv-be-transmitted-via-mosquitos), and nor can other highly virulent viruses that are transmitted through blood and bodily fluid exchanges, such as Ebola (thankfully!).

Marcus Junius Brutus asked an interesting question in a comment on that above-linked question's answer:

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> [Why can't] tiny droplets of blood remain on the proboscis of the mosquito long enough for said mosquito to inject its proboscis into the bloodstream of another person? In particular, in what physical aspects is the proboscis different than a needle or razor? I am not talking about blood sucked into the gut of the mosquito, I am talking about blood remaining on the proboscis
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There's no doubt that dirty mosquito proboscises *aren't* a vector for blood-based viruses (if they were, Ebola would be completely uncontainable in West Africa), and I understand the explanation on the above linked answer that there's no route from the mosquito's stomach to its proboscis other than via saliva production, but given how tiny the amounts of blood are that are required for transmission of such highly virulent viruses, why does it never happen in a similar way to transmission from a dirty needle or razor? 

I've never seen a mosquito put its proboscis in sterilising fluid or an autoclave between feedings... 

My best guess would be that mosquitoes are very efficient at taking up all available fluid, and that surface tension means that at this scale it is possible to take up every last droplet - but that's just a guess.