Good questions. I answered them in two parts for your group. Hope this helps!

Bananas
Bananas are grown throughout the tropical areas of the world (over 100 countries), and are native to tropical areas of South and Southeast Asia. They are used for fruit, but also their fiber and as decorative species. Bananas are a very important crop globally, and certain breeds are used almost exclusively for food (the one we eat is called Cavendish). Because the bananas we eat dont have high genetic diversity (they are related to each other like a family), they are affected by a number of diseases that can wipe out the possibility of growing an entire banana type (this happened to the Gros Michel breed that your grandparents would have eaten as children, but is no longer available commercially).

Banana diseases include a number of variants on a fungal pathogen known as Panama Disease. It was first seen in the 1900s and spread to banana farms across the world by the 1950s, wiping out the Gros Michel. This fungus kills banana plants by which getting into the plant through its roots and moving wit water through the trunk and leaves. It produces a gel and gum that stops the flow of water and nutrients through the plant, starving it. Other strains of fungus kill banana plants by turning their leaves black and stopping photosynthesis. Banana crops have also been hurt by viruses that cause them to produce less fruit.

The diseases that affect bananas spread because bananas are reproduced from the stalks of other banana plants, not seeds. So if one plant is infected, many of them will be as the plant is propagated (they do not have genetic resistance, because they are all very closely genetically related). Diseases also spread when fruits are shipped, and from little bugs called aphids, which spread viruses.

Although people can get sick from fungal or viral infections, they arent the same ones that hurt bananas, so we dont get sick when the plants do.

Fish
Fish are an important part of peoples diets worldwide. Currently, fish stocks are dramatically declining from over fishing, so its a good idea not to eat fish without checking if they are sustainably fished 1st. The Monterey Aquarium puts out a nice guide available here:

Besides being bad for fish populations, over harvesting also can affect the environment of fish and shellfish that themselves eat these over harvested species. This can lead to fish and shellfish that are more susceptible to disease (like mycobacterial lesions in rockfish) that may be contagious to people whom are fishing.

As for fish diseases, the most common problem is toxins in the environment that accumulate in the fish as they grow. These things cant spread like bacteria or viruses, but theyre not good for people to eat. These toxins include heavy metals (from peoples garbage, usually), like mercury and arsenic, and chemicals produced by people that end up in the water, including: polychlorinated biphenyls (PCBs) and the pesticide DDT.

People can also get sick from eating shellfish contaminated with pathogens from human and animal fecal waste (poop) that comes from city runoff, wildlife, wastewater treatment, agricultural runoff, and boating and marinas.Toxins produced by certain types of algae can contaminate marine fish and shellfish as well. These toxins accumulate when algae is growing well, usually because of farm fertilizer run off into the ocean. These toxins not only can really hurt fish communities sometimes causing very big numbers of fish deaths that can also affect seabirds, marine mammals and humans.

When we eat fish that are contaminated, this bad stuff can build up in us too. Thats why there are signs in coastal areas reminding people that the things they spray on their lawns and trash they leave on the streets, all runs into the ocean.

I'm not sure what bananas and fish have to do with each other, but this sounds like a fun project!! Bananas are plants while fish are animals, and each is affected by their own set of diseases. Let's start with bananas. The diseases that make bananas sick most often affect the plant itself and not just the fruit part that we eat. Banana plants can get sick from all sorts of things: bacteria, fungi, viruses and parasites. Besides infections of the banana plant, bananas can get sick from a lack of nutrients (food, minerals, water) and from changes in the environment (too hot/cold, change in light). Most banana diseases are caused by fungi. In fact, there is a very fungus causing a sickness known as Panama Disease that enters banana plants through their roots and stops the flow of nutrients throughout the plant. If this disease keeps spreading, we may not have bananas in as little as 10 years!

Fish are animals (like you and me), and so there are a lot of diseases that affect fish. Fish can also be infected with bacteria, fungi, viruses and parasites (like little worms). Changes in diet and environment can also make fish sick. There are many diseases that fish can pass on to humans and make us sick. Fish can get infected with Salmonella, a bacteria that causes food poisoning in people, and so if we eat a sick fish then we too can get sick!

Your question is a very important one! The way our food is carried from one place to another before we eat it is very important. If our food is not packaged correctly then we can all get very sick. Most food items that are perishable (meaning they can go bad)- which includes bananas and fish- are kept very cold. Microorganisms (bacteria, viruses, fungi, parasites) that can cause disease have a hard time growing at very low temperatures and the germs will not be able to spread. There are special refrigerated trucks used to keep food cold while moving long distances. Even if it is kept cold, you never know if there is a disease-causing microorganism on your food. Washing your fruit (including bananas) will help get rid of germs that can make you sick, and the best way to safely eat fish is by cooking it to a high temperature that kills these nasty germs.

I hope this helps with your project, and let me know if you have any more questions. Good luck!