What is the stance of Islam regarding medical examinations before or after marriage if they reveal infectious diseases? Is it permissible to refuse marriage, separate, or prevent pregnancy based on medical opinion regarding the potential for children to be afflicted with diseases? What is the evidence for this from the Qur’an and Sunnah?

The default is that there is no need for premarital medical examinations for someone who is healthy and free from ailments, while having a good opinion of Allah. However, if diseases become widespread in society due to a lack of chastity, then examination becomes justified. The basis for marriage is choosing a person of good character and religious commitment; otherwise, it may be easy to falsify results. Each spouse must inform the other of repulsive or contagious defects and diseases; concealing this is deception and a sin, and the other party has the right to annul the marriage. The majority of scholars held that defects necessitating annulment are of two types: those that prevent intercourse, and those that are repulsive or contagious. Others, like Ibn al-Qayyim, expanded this to include every defect that repulses the other spouse and does not fulfill the purpose of marriage. If one spouse knows of their illness and informs the other, and the other consents, then their marriage contract is permissible, even if it is said that it will be transmitted to the children, because it is speculative. However, if trustworthy doctors confirm that the offspring will be born with a hereditary disease to a degree that prevents them from living a normal life, then there is no harm in not marrying, or in separation afterwards if they desire. Temporary contraception is permissible. Al-Shafi'i stated that leprosy and vitiligo are defects that annul marriage due to their transmission to the child and offspring.