How does the Curse work?

In this game, Billie, played the influenza virus.
The goal of the influenza virus is to survive and grow.
It accomplishes this by moving into as many populations
as it can.

This virus exists out in the wild, in populations of wild pigs
and birds such as ducks... and from time to time
it enters human populations when they have contact with
these wild animals (or when the virus infects domestic
animals, and humans interact with the domestic animals).

Influenza doesn't automatically move from population to
population... but it has three attributes that enable it to be
pretty efficient at invading populations.

1. Influenza is extremely contagious,
and can spread in the air from host to host. It is not
unusual for a flu strain to sweep through an
entire population.

2. Influenza is not stable in structure. It produces new
strains quite readily, especially when when two or
more established strains move into the same population.
This helps the virus, because strains have often evolved
to suit one animal type, and thus have trouble jumping
the "interspecies barrier" into another type of animal.
Also, populations evolve, and will develop resistance or
immunity to any one virus strain.

3. It doesn't usually kill its host. This helps it spread
within a population.