Trees are one of the hardiest plants on the planet. You can still find trees that have been alive for thousands of years. Many can trace their heritage back to the first plants to appear on the planet. However, as the millennia have rolled on, trees are just as likely to succumb to illness and disease just as they have always done. In recent years the scourge of the Dutch Elm disease removed this native species to the point of extinction. There are now just a few pockets of them left. The same is coming true again with the appearance of Ash dieback. Small mites poison and kill the tree before moving on to the next one.
One of the first signs of your tree having a disease is dead branches or an unnatural increase in dormant limbs. You have to make sure that the tree has these removed, professional for preference, and then you can feel more assured about its condition. This will mean a fair bit of waste but, with the right Trailer Parts from a supplier like https://autoandtrailer.com/shop/trailer-parts, for example, you can have these branches removed in no time.
Other signs of illness include a rusting or discolouration of the leaves. Buds also seem to be less frequent and stunted. Keep an eye out for any large number of pests and beetles. This can be carriers of the tree diseases, or even the problem themselves.