Clean water and nutrition are factors in maintaining a healthy body.
Your assertion about healthy people not being susceptible to disease is wrong though. Healthy people are just as likely to contract disease, but they have a better chance of having better outcomes, than those that are unhealthy.
With some diseases, such as influenza, having a healthy immune system, can be detrimental, rather than a benefit.
Anyone interested can have a look at the death rate among healthy individuals, during the 1918 spanish flu pandemic.