Which type of immunity function in the body when encountered by a one type of virus/bacteria..?

I have completed masters in microbiology and i'm learning immunology since my graduation and was one of the tough puzzle to understand and its behind an imagination...but a good subject...there are three types of immunity in our body... 1) Humoral immunity/specific/adaptive 2)Non specific/innate imunity 3)... show more I have completed masters in microbiology and i'm learning immunology since my graduation and was one of the tough puzzle to understand and its behind an imagination...but a good subject...there are three types of immunity in our body...
1) Humoral immunity/specific/adaptive
2)Non specific/innate imunity
3) First lin eof defences which includes skin,mucous,cillia etc..
my question is...
A) If a virus/bacteria enters into our body,which of the above type is chosen to destroy pathogen...or if all are chosen or they are random..?
B) They say if virus/bacteria has to cause disease they must passs through all defences...then why do these defences/strategies fail during disease conditions..?
share ur word on this..
c) If cytotoxic T cells kills oncogenic tumours/cells then still why do cancer occurs..?
thnx in advance...
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