Where it starts:
http://en.wikipedia.org/wiki/Tommy_Westphall
How it spreads:"Tommy Westphall (portrayed by Chad Allen) is a minor character from the television series, St. Elsewhere. This minor character took on major significance in the series' final episode, "The Last One".
Tommy was the autistic son of Dr. Donald Westphall (Ed Flanders), first appearing in the show's second season. In the final scene of the final episode, it is revealed that the entire series is a figment of Tommy's imagination - and his father is not a doctor in a hospital at all. The closing image is of St. Eligius hospital, the setting of St. Elsewhere, inside a snowglobe that Tommy owns."
.St. Elsewhere is directly linked to Tattingers, The White Shadow, Method & Red and Homicide: Life on the Street because of one or more of its characters appeared on those series.
Characters from The Bob Newhart Show and Cheers appeared on St. Elsewhere, the latter thereby creating a link to the spin-off Frasier. Newhart was a dream had by a character on The Bob Newhart Show and thus also directly linked. The lead character from the Bob Newhart Show, Dr. Bob Hartley, also appeared on an episode of Murphy Brown.
Homicide: Life on the Street is directly linked to Law & Order, Law & Order: Special Victims Unit, The Beat and The X-Files because one or more of its characters appeared on those series.
Characters from St. Elsewhere, Oz, Chicago Hope and Law & Order appeared on Homicide.
All of these series can, through Munch, also be linked to The Simpsons as The X-Files characters Fox Mulder and Dana Scully appear in an episode of that series. In addition, there also exists indirect links to the X-Files feature film, the spin-off series The Lone Gunmen, sister series Millennium, and the short-lived series Strange Luck which made reference to Fox Mulder in one episode
This eventually hits over 200 shows:
http://en.wikipedia.org/wiki/A_List_of_ ... l_Universe
Missing from that list is South Park, which now has to be included since Bart was on, I think, the Family Guy episodes.
Edit: It also includes all the David E. Kelley shows (Boston Legal, Ally McBeal, Picket Fences, Boston Public, etc).
