Does the mention of seventy thousand angels seeking forgiveness for a Muslim when he visits the sick contradict the Hadith "An ill person should not come to a healthy person" and the Prophet's (peace be upon him) prohibition of mixing with lepers?

There is no contradiction between encouraging visiting the sick and avoiding those with contagious diseases. The texts can be reconciled by understanding that if a patient's illness is contagious, visiting them is not prescribed if contagion is feared. This is supported by the Prophet Muhammad's guidance (peace be upon him) on guarding against contagious diseases and his instruction to avoid those afflicted by them, as in his saying: "Flee from the leper as you flee from a lion." It is possible to connect with a patient from whom contagion is feared by inquiring about them without getting too close or touching their clothes. The forbidden mixing refers to what directly causes contagion, such as sitting on their bedding, and not merely proximity or looking.