**2. Basis of infectious disease**

Spread of an infectious disease depends on several factors like factors related to the pathogen, environmental factors, population factors and finally, social structure and behavior of the contagion. Natural birth and death rates of the population are independent of the pathogen. The rate of transmission may differ in different age groups where incubation period may vary. Type and mode of contact or incidence function and immunity duration are known to govern the spread. Geographical location and seasonality are other factors affecting the spread of an infection [2-4].

Complete understanding of infectious diseases requires knowledge of the various processes involved in host-parasite interactions. The two most important processes in these interactions are the epidemiological process associated with disease transmission within the population and immunological process involved in the disease dynamics within the host. Modeling of host-pathogen interactions helps in identification of key factors that may have a major impact on the outcome of an infection. In dynamic models of viral diseases, epidemiological modeling is based on the interaction between the susceptible and infected classes of the population and within-host dynamics is usually overlooked [5]. In any viral or bacterial infection, one of the key determinants of the disease progression within the host is the immunological status which is governed by dynamic interaction between different groups of cells and various signaling molecules [1].

Some of the terminologies associated with infectious disease are (a) infectivity or secondary attack rate, which is defined as the ratio of number infected and number exposed, (b) patho‐ genicity or illness rate, which is expressed as the ratio of number with symptoms and number infected and lastly, (c) virulence which may be written as the ratio of number of severe/fatal cases and total number of cases. For example, chicken pox and measles are characterized by high infectivity, high pathogenicity and low virulence whereas, smallpox is recognized by high levels of all the three variables and tuberculosis is found to possess low infectivity, low pathogenicity and high degree of virulence.
