  Pathogen outbreaks (i.e., outbreaks of bacteria and viruses) in hospitals can
cause high mortality rates and increase costs for hospitals significantly. An
outbreak is generally noticed when the number of infected patients rises above
an endemic level or the usual prevalence of a pathogen in a defined population.
Reconstructing transmission pathways back to the source of an outbreak -- the
patient zero or index patient -- requires the analysis of microbiological data
and patient contacts. This is often manually completed by infection control
experts. We present a novel visual analytics approach to support the analysis
of transmission pathways, patient contacts, the progression of the outbreak,
and patient timelines during hospitalization. Infection control experts applied
our solution to a real outbreak of Klebsiella pneumoniae in a large German
hospital. Using our system, our experts were able to scale the analysis of
transmission pathways to longer time intervals (i.e., several years of data
instead of days) and across a larger number of wards. Also, the system is able
to reduce the analysis time from days to hours. In our final study, feedback
from twenty-five experts from seven German hospitals provides evidence that our
solution brings significant benefits for analyzing outbreaks. It is also
applicable to COVID-19 hospital-associated transmissions.
