What will happen to humanity if it finds a way to get rid of all diseases?

Well, if you imagine, then it seems to me that at the state level they would come up with such a scheme that for each offense they would take a certain amount of life time, and this time would be determined by your income, professional activity, merits to society, education, number of vom children, altruism and all that. Maybe they even introduced a new currency, measured by your time.

I think there is even a movie on this topic.

Utopia. A person will find a way to get rid of this method, It's hard for people to learn to be correct and spiritual, most likely they will come up with a way to destroy each other

The disappearance of diseases will entail a change in human consciousness, and therefore in the entire human culture - thinking, religion, films. Everything from sayings to the Meaning of Life will undergo rethinking, since the factor of sudden (or simply premature) death from an illness had a great influence on our thinking and perception. It will be an interesting time!

To discuss further, additional conditions are needed, such as: this was done by a hypothetical omnipotent wizard or a man, if the latter, then this entails an incredible leap in the entire scientific field and here I personally would be most interested in resettlement in space, artificial intelligence, cloned artificial people ... etc.

The ideal answer to this question would be Jack Vance's novel, Eternal Life. There, of course, in connection with the general physical immortality, there was a problem of lack of resources. And then at the - government level - a decision was made: to calculate the average acceptable life expectancy for everyone. After the expiration of the term, a special official-killer came to the person, who took him to execution - completely painless. For some SUPER services to society, one could deserve an extension of life (Therefore, 10 years before death, everyone began to fuss and think about what to do "something like that"). But the same period was taken away from the rest, dividing it by the entire population (it turned out that each lost a fraction of a second of his life). Naturally, we are all human, and frauds began on the part of the rich and those in power in society. They began to increase their terms, and the life of the people as a whole began to shorten (as the planetary computer system informed them about). Until the time the people endured, until the main character of the novel - in his personal interests - did not make a callapse. Namely, he got into the system and made it so that the elite received CENTURIES of extra life). And then the common people saw that their life was reduced by 5-10 years. What happened next is clear. First, they began to beat ordinary officials-killers. Further more. As a result, the entire system was broken and the romance ended there.

You can, of course, fantasize on this topic in different directions from eternal happy life to self-destruction, as in Julia's answer.
But I, as a critical realist, will express my opinion without pretending to be fair, but admitting the probability this development of events:
Even if humanity finds a way to get rid of all diseases, it will also find a way to hide this breakthrough from the people. Perhaps billionaires and presidents will suck on the miracle serum on weekends, but ordinary people will not know about it. Think for yourself! Medicine and pharmaceuticals are one of the largest, most influential and profitable industries in the world. I do not deny the possibility that even now there is already a cure, for example, for cancer, but it is not profitable to launch it.
Leave the house and look around. What do you see? Basketball court, swimming pool, healthy plant food shop? No. Most likely, not even one, but several pharmacies a step away from each other. Why are all sorts of drugs advertised on TV and not a word about research related to the influence of lifestyle and diet of people on the occurrence of serious diseases? Why is veganism rejected and humiliated by all means, and gluttony and drunkenness (Eat-drink everything, yes, as much as possible! "Mezim" will help you! Until one day a blood clot in your clogged vessel breaks off and kills you) is promoted in all advertisements? There is a great variety of examples of such propaganda, flushing, disinformation and censorship. Why is this all? Because NOBODY! NOTHING! does not do for the sake of PEOPLE. Money rules the world. And the interests and influence of those who have this money are taken into account, in particular: animal husbandry and pharmaceuticals.
Read any of the many articles and statistics on the impact of animal husbandry on the environment (spoiler alert that it takes the lead in pollution of Mother Earth) and the use of animal products (meat, eggs, milk and everything that is produced from it) on human health ... Everything is crystal clear, accessible, proven and obvious. But no one in our country will approve of your decision, for example, to go vegan. The doctor (DOCTOR!), Like a granny near the entrance, will only sing condemningly, disregarding the facts.

I got carried away. I think that's enough))
Please do not get involved in unrelated anti-vegan controversy.

This is a utopia, even if all existing ones disappear, it is more likely that such a disease will appear that they will not have time to stop, or it will be impossible to stop.

Life expectancy will increase, mortality will decrease, which will lead to an overpopulation of the planet, and, as a result, a deterioration of the environment, a shortage of drinking water, food and useful resources, which will undoubtedly become a prerequisite for all sorts of terrible things: war, cannibalism, anarchy - of course, these the consequences will not immediately appear; nevertheless, if science in such conditions does not invent anything, then everything will end tragically. What happens if this situation occurs in the distant future? It remains only to fantasize, you can write a science fiction novel like that.