  Over the past 40 years, the massive LBV/WR system HD 5980 in the Small
Magellanic Cloud has undergone a long-term S Doradus type variability cycle and
two brief and violent eruptions in 1993 and 1994. In this paper we analyze a
collection of UV and optical spectra obtained between 1979 and 2009 and perform
CMFGEN model fits to spectra of 1994, 2000, 2002 and 2009. The results are as
follows: a) The long term S Dor-type variability is associated with changes of
the hydrostatic radius; b) The 1994 eruption involved changes in its bolometric
luminosity and wind structure; c) the emission-line strength, the wind velocity
and the continuum luminosity underwent correlated variations in the sense that
a decreasing V$_\infty$ is associated with increasing emission line and
continuum levels; and d) The spectrum of the third star in the system ({\it
Star C}) is well-fit by a T$_{eff}$=32 K model atmosphere with SMC chemical
abundances.
  For all epochs, the wind of the erupting star is optically thick at the sonic
point and is thus driven mainly by the continuum opacity. We speculate that the
wind switches between two stable regimes driven by the "hot" (during the
eruption) and the "cool" (post-eruption) iron opacity bumps as defined by
Lamers & Nugis (2002) and Gr\"afener and Hamann (2008), and thus the wind may
undergo a bi-stability jump of a different nature from that which occurs in
OB-stars.
