I do not agree with this. Sometimes truth is too hard to accept as is. And hard truths can be better digested through metaphors. It allows people to understand something that scares them. Now this is not to say that I believe we should conceal the truth, but an experience of illness or cancer could be better explained through metaphors. It can allow other individuals to better understand illness and accept it as a fact of life. So as long as you do not create any misconceptions through metaphors, I do not see why they are a bad thing.

Why are people more afraid of illnesses like Cancer and TB.

I don't really agree with this. I think it is shameful especially if you are a younger individual. People try to explain it as 'drugs' or 'unhealthy diet'.

that reminds me of the movie Philadelphia with Tom Hanks.

Perhaps because of years of evolution the human brain learnt to avoid people who look different/ ill as a matter of survival. Maybe in the next couple of thousands of years of further evolution with the knowledge of medicine we could adapt to a stage where we do not fear those who are ill (or those who are different).

Kafka has also done similarly in 'metamorphosis' where he shows the struggle of clearly mental illness through the unpleasantness of a cockroach. When George (I believe that was his name) wakes up to find out he has turned into a cockroach and further indicating that the cockroach is a metaphor we see the alienation from his family. They are afraid to get close to him and George slowly feels less and less like himself.