Many have argued that seeking the services of a physician is a form of heresy.
If God ordained that someone should be ill, how can you intervene and try to cure him?
Likewise, if God ordained that someone should be poor, how dare you interfere with His doings?
The answer is that although God indeed decides that some people should fall ill and some people should be poor, there is no requirement to preserve that reality.
Man is permitted – even required – to intervene.
Even Rabbi Nachman of Breslov, who denounced physicians in the strongest terms, saying that when the Angel of Death understood that he could not kill everyone by himself, he appointed the physicians to do it for him (Sihot HaRan 50), did not oppose medicine perse.
He himself claimed, on another occasion, that a father who does not vaccinate his son against smallpox is a murderer.
Apparently, his opposition to physicians did not stem from a conception that it is forbidden to interfere with God’s doings, but simply from his deep distrust of the physicians of his time.
In a certain respect, he was truly justified in this distrust.
–Rabbi Adin Steinsaltz
From Talks on the Parasha