ABSTRACT
Communicable (or infectious) diseases comprise clinically evident illness (i.e. characteristic medical signs and/or symptoms of disease) resulting from the introduction, presence and growth of pathogenic biological agents in an individual host organism. In certain cases, infectious diseases may be asymptomatic for much or their entire course. Infectious pathogens include some viruses, bacteria, fungi, protozoa, multicellular parasites and aberrant proteins known as prions. The term ‘infectivity’ describes the ability of an organism to enter, survive and multiply in the host, while the infectiousness of a disease indicates the comparative ease with which the disease is transmitted to other hosts. An infection is not synonymous with an infectious disease, as some infections do not cause illness in a host (Figure 9.1).
