I want to count the cooccurrences of a disease pair or a disease-gene pair in all pubmed abstract to find evidence for the links. I have many such pairs and want to do an extensive search. Is there an easy way to do that? I am thinking to download all Pubmed abstracts and do the search myself. Can anybody let me know the best way to download all Pubmed abstracts (up to a limit, say last 20 years)? Since I only need abstracts, which are public, I think there should be easy ways but I couldn't find it on google. Thanks!