there have been is the present perfect tense of there are.
Present: There are more than 700 cases ... Past: There were more than 700 cases ... Present perfect: There have been more than 700 cases ... Past perfect: There had been more than 700 cases ...
The there is construction points out the existence of s
"There have been" differs for "There are" in that if the incidents have been noted, they may no longer exist: "There have been more than 700 incidents of swine flu in Montgomery county" doesn't mean that ALL 700 are present now. If you used "There were" or "There had been", the cases no longer occur; but in the use of "There have been", an outbreak of swine flu may continue to occur at various ti