In the following sentence, should there be an article 'the' before 'dengue virus'?
According to WHO, 2.5 billion people are at risk from the four major types of dengue virus, which is transmitted to people from infected mosquitoes.
The sentence is correct as it stands.
You may insert the word "the" before "WHO" (a quick google survey shows both "according to WHO" and "according to the WHO" being used. (in speech one would insert a "the" if read as W. H. O but not if read as "hoo".)
You shouldn't use an article before dengue virus. In expressions like "two types of apple" or "72 breeds of sheep" or "both makes of computer", the word being classified doesn't normally require an article. Exceptions to this are when the word being classified usually has the definite article "types of the 'flu" is correct, because "the 'flu" is used idiomatically with the article.
