Because her immune system is weak, she catches a cold easily.

This virus can be transmitted from human to human.

Retroviruses can have serious effects on the human immune system.

His actions exerted a great influence on our plans.

He is living as one person.

This park is always bustling with people.

This park is overflowing with people.

This problem is very serious.

Do you understand the seriousness of this problem?

His opinion had a big influence on my decision.