Quote Originally Posted by oldboy
This is out of BBC new Bangladesh water may still be unsafe:
Ministers say that more research is needed on this and that people should not be alarmed.
Does 'more research is needed on this' mean 'it's necessary to research this (=the problem above)' or anything else?
Yes. It means it's necessary to research this (more).

Quote Originally Posted by oldboy
The water that these wells tapped was contaminated with arsenic, and millions were affected in what the World Health Organisation termed 'the greatest mass poisoning in history'.
affected = influence, touch
in = by
That is '...millions were affected in what...' = 'millions were influenced/touched by what'?
I can't understand sense of that part of the sentences.
The "what" (noun) you are looking for is "poisoning." The basic idea is:

...millions were affected by the poisoning.

(You are correct about the meaning. It means millions were influenced/touched by the poisoning. Even simpler: millions were poisoned.)

The rest of the sentence describes the poisoning. You can think about it like a который clause:

...millions were affected in the poisoning, which the World Health Organisation termed 'the greatest mass poisoning in history'.

(This means: The poisoning was so bad that the World Health Organization termed it (called it) "the greatest mass poisoning in history.")

So the original part ...in what the World Health Organisation termed... is just a way to tell us more about the "what" (noun = poisoning).

Does that help?