Infections are caused by bacteria, virus, or disease of a genetic predisposition. Infections are, in fact, the evidence that the bacteria or virus or genetically linked disease is present and doing quite well. This evidence can take the form of a cough, runny nose, fever, pus, swelling, blood, and many other symptoms. You have seen many of them, I am sure.
For human beings, bacteria or virus is contracted from another living host. Disease can even be caused by a genetic predisposition that may activate during a person's lifetime.
Prolonged growth of bacteria or virus or genetic disease within the human body can lower the resistance to other bacteria and viruses. And, prolonged, constant exposure to pollution can only exacerbate.... or make worse.... the symptoms of the existing bacteria and/or viruses or provide an invitation for additional bacteria and virus to enter the body in its weakened condition.
The question that was given to you was actually an issue of the biological sciences. However, in our world today, the social sciences are being aligned with the biological sciences.
You must make the decision for yourself as I am of the generation that may "think" differently than your generation. Your generation may come to agree with the findings of my generation and the generations before.
I remind you that Stephen Hawking once said,
"Intelligence is the ability to adapt to change."
The world is dynamic, as is life. You must answer this question as you "see" the world today.
Remember to write your answer with the evidence as to why you hold that opinion. This evidence must be very clear so that anyone may be able to understand your opinion. Also, what you write will be memorialized. That means that it will remain... sometimes long after you even remember that you have written the words. So write very well. Your words will be speaking for you.
Good luck. And, if you would be so kind, I would like to hear the response of your teacher as I find this changing world to be absolutely fascinating.