Outcome of infection following heterologous viral infections is dependent on the stages of ongoing immune response to a previously encountered pathogen:
(A) If the infections with the viruses (here for example 2 virus system) occurs at the same time, the outcome could be enhanced immunopathology possibly due to the immune responses to both the viruses reaching the peak at the same time.
(B) Prior infection can result in activation of APC and the new incoming infection at this stage encounters mature APC, efficient antigen presentation and faster disease progression. Also the new incoming pathogen creates a strong antiviral state that might result in reduced viral loads of first pathogen.
(C) Upon activation APC’s secrete cytokines that results in T helper subset differentiation and the incoming pathogen at this stage encounters a polarized immune response. Encounter with Th1 type of immune response can provide bystander protection or enhanced imunopathology whereas T regs
(D) can result in suppression of immune responses to incoming pathogen, which may be protective or pathogenic. The outcome of heterologous viral infections however, depends on the type and sequence of viruses.