Infectious diseases are caused by pathogens. Bacteria, viruses, fungi, and worms/helminths are common examples. Disease symptoms may not appear in normal circumstances when the host's immune system is fully functional. An infectious disease develops when the immune system of the host is impaired or when the infectious agent overwhelms the immune system. Infectious diseases can affect everyone. People with a weakened immune system (one that doesn't perform as well as it should) are more susceptible to certain infections. Infectious diseases are a major cause of morbidity and mortality worldwide.