Answer:
Assuming you mean deseases, we can take an example from the bubonic plague, which killed 50 million people in Europe, it was actually carried by fleas which were carried around by rats and then jumped on people, because Europe was extremely dirty and most lived in poverty, desease was rampant and there wasn't much you could do about it. However deseases can simply just happen, like the COVID pandemic, it's believed that it was transmitted by a bat to a human who then travelled back to their home country and spread it to others.
Explanation:
Desease can emerge suddently, depending on poor sanitation, overcrowding and changes in handling and procesing large amounts of food. Or simply get transmitted from humans to animals.
