Why is it difficult to produce vaccines for viral diseases such as the adenovirus or HIV?

Why is it difficult to produce vaccines for viral diseases such as the adenovirus or HIV?Why is it difficult to produce vaccines for viral diseases such as the adenovirus or HIV?
I have some clue for the reason why vaccine doesnt work on HIV, but i have no clue at all for adenovirus ( cold)..
so please help!

Biogirl
The problem with HIV and adenovirus is that they mutate so quickly. Their genotype changes so often that they begin to act on different proteins in their host's body. It makes it impossible for researches to come up with a vaccine that will work for all cases of the virus.

Christine
They evolve faster than modern medicine does.

mamabear1957
Seems once researchers find some part of the virus that they can attack with a vaccine, the little boogers change or mutate so that the vaccine doesn't work.

YodasTatoo
Not enough funding and the drug manufacturers are lobbying to keep it that way. As long as AZT and other drug and drug coctails keep people alive with the virus, if a cure comes along, they lose out on the profits.

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