A disease is something like cancer, heart disease, or dementia. There are medical long-term illnesses caused by pathogens or genetic disorders. Obesity is most often caused by a lack of motivation and unhealthy food. Something caused by factors like that cannot be a disease. What if someone is obese because they are paralyzed and cannot exercise? Or if someone is taking medicine that causes weight gain? These are not things that are considered the beginning of a disease.
Diseases like cancer can be caused by outside elements (e.g. carcinogens). Just because obesity is caused by things like diet and exercise doesn't mean it cannot be classified as a disease.
[P1] A disease is not something that can be caused by external factors like injury or diet.
Rejecting the premises
[Rejecting P1] If cancer can be caused by outside forces such as carcinogens, then obesity can be classified as a disease.