IELTS writing. Is this sentence too long and unreasonably arranged?

Two things, first of all your use of the word concernedly, I have never come across concern used as an adverb, not saying it can’t be used, but I would not use it here. If you need to include something I’d say worryingly, but personally, I think it’s better without.

The phrase which carry highly contagious doesn’t mean anything.

What are they carrying that’s highly contagious? You’ve not said.

You need to say either which are highly contagious, or carry highly contagious pathogens or something similar.

I would also split before the word despite, so the second sentence would be This is despite the ...

Thanks for helping me 🤩

So is it better if I say :
“Many reports warn that some infectious diseases, such as cholera, malaria and tuberculosis are worryingly increasing in the number of fatal cases, despite these diseases being preventable and curable.”

So this would be “ This is despite the fact that these highly contagious diseases are preventable and curable”
Is that ok? 😊

Yes it would be, but if it was me I would drop worryingly as well.

...malaria and tuberculosis are increasing in the number of fatal cases,

That’s me, and we’re talking about differences in style, not accuracy, there’s nothing wrong with your example.

That's fine.

Read your own sentences out loud. If you get winded while reading them, then they are probably too long.

Thanks a lot for your help 🥰