Harmful synergistic interaction of biological agents

Compound disease
Occasionally two biological agents combine to produce a more severe disease than either of the agents alone would produce. This synergistic interaction may be responsible for severe epidemics. It increases the unpredictability which accompanies the introduction of any new biological species into a new environment. Multiple-factor agents represent a new possibility in biological warfare, since two relatively harmless agents can be kept separate until released.
It is thought that the pandemic of 1918 in which influenza led to the deaths of many civilians and soldiers (more American lives were lost due to this disease than to gunfire) may have been such a compound disease.

Just when bacterial resistance to antibiotics is exceeding the capabilities of the chemists to produce more effective drugs, the increased incidence of UV radiation on the Earth's surface is weakening the immune system strength of living organisms.

(C) Cross-sectoral problems