  We report the serendipitous discovery of VVV-WIT-12, an unusual variable
source that seems to induce variability in its surrounding nebula. The source
belongs to the rare objects that we call WITs (short for What Is This?)
discovered within the VISTA Variables in the V\'ia L\'actea (VVV) survey.
VVV-WIT-12 was discovered during a pilot search for light echoes from distant
Supernovae (SNe) in the Milky Way using the near-IR images of the VVV survey.
This source has an extremely red spectral energy distribution, consistent with
a very reddened ($A_V \sim 100$ mag) long period variable star ($P\sim1525$
days). Furthermore, it is enshrouded in a nebula that changes brightness and
color with time, apparently in synch with the central source variations. The
near-IR light curve and complementary follow-up spectroscopy observations are
consistent with a variable Young Stellar Object (YSO) illuminating its
surrounding nebula. In this case the source periodic variation along the cycles
produces an unprecedented light echo in the different regions of the nebula.
