What measures do Catholic dioceses take at Mass during viral outbreaks? What changes are made at Masses around the world because of outbreaks like the Covid-19/Coronovirus outbreak? For the last two weeks I've noticed a few changes, but we don't do communion in both species on my diocese and I'm not really sure what I should expect when I go to a different diocese. 

Are priests expected to announce the differences that they do at each Mass so visitors won't be caught unawares?
What measures do Catholic dioceses take at Mass during viral outbreaks?

First of all it will depend on the severity of a particular outbreak in a particular geographical location or diocese.

For the most part, a particular national conferences of bishops would make up some general guidelines for a particular ecclesiastical region. Nevertheless a local ordinary is always free to impose further measures (within reason) for his own subjects in his own (arch)diocese during certain circumstances.

At the moment there seems to be no official statement of particular measures from the US Conference of Catholic Bishops itself, for the coronavirus outbreak.

For example, I live in the archdiocese of Vancouver and a few years ago, His Grace asked that the ”sign of peace” should be suppressed during a rather nasty flu outbreak that had hit Vancouver and adjoining areas. 

Up here, pastors generally announce any changes of the Mass itself (at least for the first time) at a convenient moment at Mass, since most parishioners do not read the weekly bulletins prior to Mass. Most bulletins will have fuller information for the faithful to peruse in their own time at home. This is how we do things here.

As a token of what guidelines are being implemented I am going to refer to what the US dioceses are doing in light of the Covid-19/Coronovirus outbreak. Canada will follow similar protocols.


  Dioceses nationwide are taking precautions to guard against the spread of the coronavirus and reminding parishioners to take commonsense steps related to hygiene in their personal lives.
  
  Among the most common preventative measures being taken are urging reception of holy Communion in the hand, suspension of distribution of the Communion cup and exchanging the sign of peace without physical contact.
  
  Diocesan leaders also asked people who are ill to refrain from attending Mass.
  
  “If you are sick, stay home until you are feeling better,” Bishop David A. Zubik of Pittsburgh said in a March 2 announcement to parishioners.
  
  In a Feb. 28 letter, Charlotte Bishop Peter J. Jugis urged clergy to remind parishioners to cover coughs and sneezes and throw away used tissues, clean all “high-touch” surfaces daily, avoid shaking hands, wash hands often and not share personal items such as cups and eating utensils.
  
  Like diocesan officials across the U.S., Jugis encouraged priests not to extend the sign of peace, distribute Communion from the cup or invite people to shake hands in greeting at Mass or other gatherings. Those who distribute holy Communion should wash their hands before Mass and clean their hands again before and after distributing Communion, he said.
  
  Similar precautions were being implemented in the Archdioceses of Washington and the dioceses of Jackson, Mississippi, and Little Rock, Arkansas.
  
  Beyond those steps, the Archdiocese of Miami suggested that parishes empty the holy water fonts at church entrances, and it also is allowing extraordinary ministers of holy Communion who feel uncomfortable in carrying out their ministry to temporarily step down.
  
  Guidelines from the Ohio Department of Health for dealing with contagious illnesses were reviewed in a statement shared with parishes and posted on the Diocese of Cleveland’s website March 2.
  
  The diocese’s Office for Worship also reminded priests, deacons and extraordinary ministers of holy Communion to continue practicing good hygiene including frequent hand washing.
  
  Archbishop John C. Wester of Santa Fe, New Mexico, updated liturgical practices introduced in January because of the severity of the flu season in Utah. In a March 3 statement, he mandated that holy Communion be received in the hand.
  
  “What is important is that we receive our Blessed Lord in holy Communion,” he said. “How we receive, while very personal to the individual communicant, is not crucial. … Receiving Communion in the hand is every bit as respectful as receiving on the tongue.”
  
  At the U.S. Conference of Catholic Bishops, Msgr. J. Brian Bransfield, general secretary, March 3 requested in an email to staff that they “refrain, until further notice, from planning any new international travel.” - U.S. dioceses adopt wide-ranging plans to limit spread of coronavirus


Other countries have taken further measures for Mass and while at Catholic liturgies and customs, as seen by the following:


  Italy has become the center of Europe’s coronavirus outbreak and Catholic bishops there have responded by cancelling Mass and encouraging the faithful to watch services from home. The Vatican has temporarily closed the Italian catacombs.
  
  The changes to Catholic worship practices are overlapping with the observance of Lent, a 40-day period of reflection and renewal before Easter. Near the end of Lent, many Roman Catholics mark Jesus’ death on Good Friday by kneeling and kissing the cross.
  
  In the Philippines, bishops have advised parishioners to instead genuflect and bow to the cross on Good Friday. It remains to be seen if U.S. bishops will follow suit. - Catholic Churches Are Emptying Holy Water Fonts Over Coronavirus Concerns