@fify-- I think there are a few microorganisms that have been able to evade cellular immunity, like HIV. HIV invades cells and changes its genetic makeup. In a way, it reprograms cells so that it can survive and replicate quickly.
Technically, when this happens, cellular immunity will destroy that cell, but somehow HIV prevents this from happening. Either it replicates too quickly for immune system cells to keep up with it or it somehow prevents immune system cells from functioning. HIV is also known to mutate, so the immune cells probably have difficulty recognizing it as well.
So cellular immunity is not unbeatable, it is not as effective as we think. Some viruses can and are able to beat it.