Question

a seminar on why do we fall ill

Answer 1

We can fall ill due to several factors. These are classified into-

Immediate Causes- These are the immediate source of disease that infects you as soon as it reaches the target organ. For. e.g Pathogens.

Several pathogens like Viruses, Bacterias, Fungi, Protozoas, and worms spread diseases. 

Also, Genetic Abnormalities is another immediate cause of any disease.

Contributory Causes- These are the sources of disease that contributes in making you fall ill. These can include the Environmental conditions, Personal hygiene and several other factors. 

So, As we see there are many factors that can make us ill. Preventing these is better than curing. Preventive measures may include Vaccines and immunisation. Also, maintaining proper personal hygiene and taking care of our surroundings. 

Answer 2

We fall ill due to factors like social environment, public hygiene, entry of pathogens etc. Many such factors combine and cause diseases. 
These are classified into contributory and immediate causes. Other causes may include improper nutrition or dehydration. :)
You can list some more such causes of diseases.