What is the difference between infected and infectious in epidemiology? I am studying the SIR model and in the infected class I, both **infectious** and **infected** individuals are included, as stated here I know that the model uses the assumption that the disease has an insignificant incubation period so infected individuals are capable of transmitting the disease as soon as they become infected. But what is the specific difference between these two terms? Does **infective** mean anything different?

(The document you link to says "see Chap. 1 for distinction between infected and infectious individuals", but I'll assume you don't have access to that - I certainly don't.)

**Infect _ed_** individuals have been infected by a pathogen. **Infect _ious_** individuals have the potential to transmit a pathogen to others.

Note that the definition given for "infected" in the source you link to is actually wrong: hosts may have a subclinical infection, in which case they are not "now sick with it".