viral background = the background that helps a virus grow?

Context:
A Fishing Expedition
Malik Peiris, microbiologist at the Univer-
sity of Hong Kong, and his colleagues,
widely credited with discovering the
SARS coronavirus, have been sequenc-
ing the viral background of bats. They
have found several new coronaviruses,
relying on sequencing machines to hunt
for novel bugs. Bats are excellent reser-
voirs for potentially lethal viruses such as
Nipah, which causes influenza-like illness
inhorsesandhumans.Ecologistscapture
bats and stick oral swabs into their
mouths to get a sample of microbes and
send these to Peiris’ lab, where scientists
use either multiplex PCR using family-
wide primers or direct sequencing to find
novel bugs.