In the past weeks, ever more dioceses around the world have cancelled Masses either in conformity with civil law forbidding public gatherings, or on their own initative to help slow down the spread of the new corona virus SARS-CoV-2. Among not a few faithful churchgowers, this step has been hard to accept. Some have criticized it, on the grounds that, in the face of hardshpis, threats to man's physical health, and the like, the faithful need the sacraments and the public prayer of the Church MORE, not less, and that, in the past, when the Church was more conscious of its spiritual mission, this was its response to epidemics. This article by Msgr. Charles Pope, and this one by Fr. Jerry Pokorsky are fairly representative of the arguments made.
Here I don't want to get into the details, but rather look more closely at the substance of the argument that cancelling public Masses can be reasonable and indeed obligatory, by reason of natural justice that prohibits risking the health of others without their consent except for a proportionate good in the same order. One might, legitimately, risk one's own health to some extent for the sake of spiritual goods (though even this can easily be excessive), one may not, except by way of just punishment, do bodily harm or induce grave risk of bodily harm to another merely for the sake of spiritual benefit.
The argument has two basic premises:
The minor premise: The gathering together of persons at Mass, even after taking the precautions that are in the power of church authorities (and even after taking the precautions that are in the power of each individual), poses a grave risk in the mid-term to the health and life of persons who have not chosen to accept this risk for the sake of spiritual benefit.
It is NOT merely a matter of the risk posed to those attending Mass; if that were to be the case, it might well be enough, at least in some cases, to ensure that those attending Mass are informed of the risk they are taking. The risk is that, as COVID-19 is infectious even before the onset of symptoms, one or more persons could be infected with COVID-19, quite possibly even without having any symptoms yet, and infect others present at Mass, and that these others will in turn infect others NOT present at Mass.
Note: If a similar risk would be present whether or not gatherings at Mass occur, the risk could not properly be attributed to the celebration of public Masses. So, if no other steps were being taken to mitigate the spread of the virus and sickness, the Mass probably could not be considered a great additional risk. The greater that steps are taken in other contexts to minimize risk of infection, the greater significance will the risk involved in the celebration of public Mass have.
The major premise: To cause grave risk to the health of persons who have not consented to accept this risk, except for the sake of bodily health or countering a greater risk of bodily harm, is contrary to natural justice and the fifth commandment.
Therefore, given the stated circumstances, for persons to come together for Mass is contrary to natural justice.
In some cases, such as in Italy or Germany, public Masses have been forbidden by political authorities. At least abstractly, the political authority, rather than the Church, is the competent authority to determine what means are required in relation to the end of bodily health of its citizens, and in this sense, given that the law is fair, not imposing an undue burden on one group as a means to the end (for example, prohibiting Masses but allowing sport events), the State does have competence to make such statutes. Of course if the state were to abuse its authority by imposing manifestly unreasonable statutes, neither the Church nor individuals would be bound to obey them. But given that the laws or statutes are in themselves reasonable or plausibly so, the Church does right, as a rule, follow them.