explain how infection might spread, how people fight off diseases, and why malnutrition can lead to the spread of disease in the time immediately following a natural disaster.

Infections spreading:
Infections are caused by bacteria, virus and other microscopic organisms. They spread through droplet (sneezing etc), they can spread through blood contact (used needles that are re-used), direct physical contact (skin infections), stool (germs from excretion) and contaminated objects.

How people fight off disease:
We have antibodies in us that fight off invading cells that contain foreign bodies. Our immune system secretes these antibodies and these antibodies will fight off the infection. The time it takes for the infection to be eradicated depends on the strength of your immune system and whether you have been vaccinated.

Natural disaster:
Natural disaster such a tsunami's and earthquakes can contaminate water supplies. In tsunami's the water from the sea will go into the drinking water which isn't safe because it could cause diseases such as cholera to spread. Earthquakes disrupt sewage structures. If these are broken, water regulation would not be as good so we end up drinking dirty water, which could also be contaminated with cholera!

Wow that was long, hope it helped! :)