This passage is extracted from a VOA article.

The researchers have developed a way to make genetic changes in the three mosquito species known to spread malaria. The changes cause their systems to attack the malaria parasite, blocking its development.

I think they used the parallel structure in the second sentence. But I think the grammatically correct sentence must be "... to attack the malaria parasite and (to) block its development."

Is my ideal wrong? If so, please show me when we can omit the conjunction "and". I think this isn't an informal writing.

Thank you very much.
The writer has chosen to create a dependent nonfinite clause, which more clearly indicates that the blocking is a result of the attack. Your version is also OK here, and 'to' is optional, since it is a parallel structure. In your version, the attacking and the blocking are two actions both caused by the change but not necessarily otherwise related.
Hi Mister Micawber,

In fact, I sometimes saw that structure, but I didn't understand their meaning.

I got it now. Thanks you very much.