There are several health issues that can lead to weight gain, a lot of which have already been listed above, so I won't repeat them. A few years ago some scientists claimed to have discovered a bacteria that made people fat, but I think it's since been dismissed as bollocks.
Thyroid and hormonal disorders are the chief culprits as far as internal problems are concerned, but a healthy diet and regular excersice can prevent the sufferer becoming obese (but they'll probably always carry more weight than if they didn't have the problem)
Thyroid and hormonal disorders are the chief culprits as far as internal problems are concerned, but a healthy diet and regular excersice can prevent the sufferer becoming obese (but they'll probably always carry more weight than if they didn't have the problem)