https://boardgamegeek.com/thread/2141748/yellow-fever
Eric Brosius:
“The idea of the Yellow Fever is that a cartel of players (typically two) bids high enough to get all of the concessions and floats all of the companies. They buy yellow trains and build track and collect revenue and allow the stock to appreciate.
However (and this is both the key point and the explanation of the name) the cartel agrees not to buy any green trains. Players who are not part of the cartel are not able to run any companies and can do nothing to bring about the purchase of a green train. Thus the game runs in the yellow phase until the bank breaks.”
Ubaldo Bastardo:
“A degenerate reply to a degenerate strategy: two of the players who are locked out of the game by the yellow fever cabal trade a single share back and forth between themselves for $1 during the stock round to force a stalemate.”