An awful illness that prevents freshers (British slang for first-year university students) from attending lectures in the first term.
May result from everyone bringing their coughs and colds from all over the country, and spreading it all about in the Fresher's Fayre and Fresher's Ball, which are always held in areas too small for the number of people trying to squeeze in. Thus resulting in everyone breathing everyone's germs and getting even more sick.
May also be a result of over-alcohol consumption in the first week.
John: Yo, going on the rugby soc pub-crawl tonight?
Tim: Can't, I feel like crap.
John: Must be fresher's flu.