For those who want to download it as a file on your computer (to play it offline, etc.), you can "right-click save-link-as" or "right-click save-as" this link: http://www.pentagonstrike.co.uk/disease.swf
I looked at the source of the page and just guessed that the flash file would be at the same directory as the html file.
(edit)
I forgot to mention. If there is no program assigned to 'play' the file after you download it, just use your browser (Firefox of Internet Explorer if you're still using that) as the program you choose to use to play this file.