A healthy lawn is the best defence against any infestation of lawn weeds, lawn pests and lawn diseases. Most of these problems which occur in lawns can only do so when the lawn or parts of the lawn are in a weak state and cannot naturally fight off any infestations on it's own. Whereas in a healthy lawn, it is far more difficult for these problems to ever establish in the first place, and if they do, they can often be overtaken and naturally controlled by a healthy lawn.
Lawn weeds will most often be first seen in bare or weak patches of turf instead of where the lawn is thick and healthy. This occurs not only when a bare patch of turf opens up to create an opportunity for weed seeds to plant and grow, but is further made worse when the natural defence mechanisms of the lawn to fight off these invaders has been weakened.
A healthy, thick patch of turf will easily crowd out, and out-compete almost all weed invaders, even if weeds do get transplanted and germinate in a healthy lawn, the healthy lawn will always keep it weak or even eventually kill the weed.
Pests such as ants in the lawn will most often never create nests inside a healthy lawn which has a healthy amount of thatch, the environment just isn't suitable for ants to live. While pests such as grubs require poor watering methods and a lot of organic material in the soil in order that they can thrive.
Also, lawns are designed to naturally survive and repair from attacks by pests, when the lawn is weakened it can no longer effectively fight off these invaders.
These examples continue with all pest types.
Lawn diseases thrive in areas that are most hospitable to them, and are least hospitable to healthy lawns. By reversing this situation and creating an area which is most welcoming to lawns, it in turn becomes inhospitable for lawn diseases.
When this balance is correct, the healthy lawn and it's suitable environment can both easily fight off infestations of disease far easier. Lawn diseases thrive in shade and having a lot of water available in the lawn thatch overnight, and lawns hate it. Begin watering in the mornings only and reduce shade on the lawn.