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if blood feeding insects can transmit many diseases(west nile,...,etc.),why cannot all infectious diseases like AIDS,Ebola be vectored by insects?

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Diseases that are spread by arthropods (I'm including things like ticks here as well as insects) are generally highly adapted for that lifestyle, and there are many specific adaptations that are needed to make this efficient. They're not simply passively carried by the arthropod; they actually replicate in the arthropod as well as in their vertebrate host, and since there is an enormous evolutionary gap between arthropods and vertebrates, the pathogen basically needs to have two, parallel and separate, lifestyles prepared.

If they don't undergo these adaptations, they'll be rapidly destroyed in the arthropod by being digested, or simply because the volume of blood is so small that unless they actively replicate there wouldn't be enough pathogen to transmit.

To give a sense of the double specialization an arthropod/vertebrate pathogen needs to undergo, here's a brief summary for tick-borne viruses:

Arboviruses differ from other viruses in their need to replicate in both vertebrate and invertebrate hosts. ... Arboviruses transmitted by ticks must adapt to the peculiar physiological and behavioral characteristics of ticks, particularly with regard to blood feeding, bloodmeal digestion, and molting. Virus imbibed with the blood meal first infects cells of the midgut wall. ... After infection of midgut cells, tick-borne viruses pass to the salivary glands for transmission during the next blood-feeding episode. To do this, the virus must survive molting by establishing an infection in at least one cell type that does not undergo histolysis. ... The infection can then persist for the life span of the tick with little evidence of any detrimental effects on the tick.

--Adaptations of Arboviruses to Ticks

So to be a tick-borne virus, these pathogens have to survive being eaten by the tick and avoid being digested. They infect at least three distinct types of cells, and have to survive the tick molting period. In the second set of cells they set up a life-long infection. And they need to do this without making the tick unhealthy, so that it can go and bite more vertebrates.

The question specifically asks about West Nile virus, which goes through a similar path of infection in mosquitos, first infecting midgut cells and then spreading to the salivary glands where it replicates to high levels (West Nile Virus: Biology, Transmission, and Human Infection).

And then after the tick bites a deer, they have to go through the whole thing again, in a brand new host that's completely different from the tick in just about every way.

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A critical point is that mosquitoes and ticks inject saliva into their host, not the contents of their stomach.

Arboviruses, including dengue and West Nile virus, are injected into vertebrates within mosquito saliva during mosquito feeding. Mosquito saliva contains anti-haemostatic, anti-inflammatory and immunomodulatory molecules that facilitate the acquisition of a blood meal.

http://ncbi.nlm.nih.gov/pubmed/18342898

So for a virus carried by an earlier host of the arthropod to infect a later host of that arthropod, it's not enough to survive in the gut, the salivary glands must be infected. This isn't possible for viruses that aren't adapted to arthropod cells.

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