I've seen cancer show up unexpectedly among two friends. One discovered she had leukemia, and underwent chemo and a bone marrow transplant. She died from a lung infection because of her weakened immune system after the transplant, not even a year after the initial diagnosis. She had just turned 27.
Another friend discovered they had an extremely rare form of mesothelioma, the asbestos-caused lung cancer, except theirs isn't due to asbestos and isn't in the lung. There are less than 300 documented cases, rare enough that there are no experts, and they're a case study. They're currently living with the uncertainty in their mid-30s.
Both friends were in great physical health (the former made the regional swim team, the latter climbs the highest peaks in North America for fun) until the diagnostic.
Seeing this has reinforced my nihilism (there is no meaning to life but the one you apply to it), and gave me greater understanding that our multicellular bodies are a peace treaty among cells that is likely to break down at any moment.