Is there a halachic precedent of healthy people avoiding crowds for fear of contracting/spreading a disease? If I recall correctly, only people that were known to be sick with Typhus didn't Daven with a minyan or go to Mikvah, but healthy people continued as usual despite the delayed symptoms of Typhus and the fact that the disease was highly contagious.

Shalom, Thank you for your question. Before turning to your question, let me firstly send my own prayers and blessings that we merit good health and a speedy end to this virus situation, for Am Yisrael and all of mankind. Secondly, let me state that whatever historic or halachic precedent we find (or don't find), it is clear that Jewish law mandates that we follow the law and advice of the local health authorities, and everyone is duty bound to make every effort to keep to the instructions issued by the local governments. We find in the Torah that people with Tzarat (loosely leprosy) had to separate from the community – as did other impure people. This could perhaps be a hint at separating contagious people – although according to our Rabbis, this impurity was a punishment for sin, and not necessarily a physical disease spread through contact. Certainly other forms of impurity were spread through contact, and we find those who were pure kept their distance from those who were impure (a Cohen cannot come into contact with a dead body etc). Although here too we are dealing with a spiritual impurity, and not a physical virus. In the examples you bring it is true that generally illness was fought by segregating the sick, and limiting their movements into society. This is being done today also, with those people who have the virus being forbidden to make contact with other people. The extra level of limiting those who are healthy from free movement is a result of the fact that with this virus this type of restriction will likely stop, or slow down the spread of the virus, rather than just quarantining those already sick – as a person in catching long before they have any symptoms (which they might not get at all). Perhaps we can find a hint to this in the Torah – on Passover night, when G-d passed over Egypt to kill the firstborn, the Jews had to stay inside, and were forbidden from leaving their houses. The Rabbis comment that “since permission was given to the destructive forces to destroy, they make no distinction between the righteous and the evil”. From here we see that when there are destructive force at large, the Jews were commanded to close themselves up in their houses. (I do not mean to imply that the present virus is in some way the same thing as the angel of death in Egypt – I certainly do venture to explain G-d's ways in the world at the moment. I only point out a source for social distancing in times of plague). In any even, as I wrote, in every age one is obligated to follow the advice of the health professionals in their location. So, as we follow the law to limit our social contacts – may we see it helping us, and the world at large. Blessings.