We know that HIV can't be transmitted by 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:
[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
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.
Answer
What you're talking about is called mechanical transmission (at least in veterinary and medical circles; plant epidemiologists call it something different). There's a nice overview by Gray and Banerjee here, although it's a bit old.
The short answer is that blood-feeding insects can mechanically transmit a range of pathogens, including viruses. Mosquitoes specifically are thought not to be great mechanical vectors because of the small quantities of infectious material that could be transferred between feeds. Large biting flies such as tabanids are thought to be much better potential mechanical vectors as they are pool-feeders; they basically chew up an area of skin and then mop up blood from the wound with their mouths. The quantity of blood contaminating a horse fly's mouthparts after feeding is reckoned to be about 10 nanolitres (Foil et al. 1987), which while not much could contain an infectious dose for some viruses reaching high titres in the blood. They also feed frequently (and painfully, which means they are often interrupted).
However, a number of viruses do seem to be transmitted in the field or in a lab setting by mosquitoes, particularly poxviruses (lumpy skin disease virus, myxoma virus and fowlpox virus, for example). Equine infectious anaemia virus (a retrovirus) is also thought to be mechanically transmitted by large biting flies in some regions despite its relative fragility. Some parasitic and bacterial diseases are also mechanically transmitted by blood-feeding insects; Trypanosoma evansi for example (the causative agent of surra) has lost the capacity to be biologically transmitted by tsetse and is solely mechanically transmitted.
(suggestions for improving this answer extremely welcome; I'm fairly new to SE)
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