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

This virus can be transmitted from human to human.

His actions exerted a great influence on our plans.

Retroviruses can have serious effects on the human immune system.

He is living as one person.

This park is overflowing with people.

This park is always bustling with people.

His opinion had a big influence on my decision.

That news greatly influenced his opinion.

This problem is very serious.