Answer:
1. The country that you are referring to might fall into poverty itself, therefore meaning that all the others in that country will be affected.
2. Low income workers (such as someone who works in a small business or essential workers) might get laid off because the business they work for simply cannot afford to pay them. If that person can't find a job, they might need to apply for financial aid.
3. If that person gets sick with said disease, depending on how deadly it is, could cause them to take leave from work. If that said person does not have enough PTO (paid time off) their company could lay them off or they could suffer severe consequences once they return to work.
These are only a few reasons why widespread disease might become a cause of poverty. I like to make my answers as straightforward as possible so that's why I only listed 3 reasons. If you would like more pls comment bellow. :)