Coronavirus, more properly COVID-19, is a virus that started in 2019 in China, mainly in the large city of Wuhan, and became a pandemic in 2020 as it spread around the world. The pandemic had an effect on many sports, including professional and amateur baseball.
The virus is also referred to as "Novel Coronavirus", and "Wuhan Coronavirus", although the latter term is disfavored. There are many types of coronaviruses, including some forms of the common cold, but the one involved here was a new type for which there was at the time no vaccine or cure. It can cause symptoms from those resembling a cold, to more severe ones including acute respiratory distress, sometimes leading to death. The virus has been particularly devastating on older patients and those whose immune system has been compromised.
The coronavirus began affecting the United States in a big way in March, 2020, during the time of spring training. By that time governments around the world had started to take drastic measures, including banning travel from certain countries, quarantining segments of the population, and canceling a number of public events, including sporting events, in order to slow down the spread of contagion.
The first baseball event to be postponed were the 2020 Final Olympic Qualification Tournament, originally scheduled for early April but pushed back to the second half of June, and the opening of the Nippon Pro Baseball season. Both of these were in Asia, where the pandemic hit first, but by mid-March, a large number of major sporting events in Europe were also cancelled or postponed, including the opening of the Serie A1 season in hard-hit Italy. In other instances, games were played with no spectators present. On March 11th, the NBA announced it was suspending its season because a player with the Utah Jazz had been infected, and the NCAA announced that its signature men's and women's basketball tournaments would be played without spectators present. Various leagues had also decided to restrict access to locker rooms and clubhouses for reporters, and asked players to limit interactions with fans, for example by banning autograph sessions. The NHL and Major League Soccer followed the NBA's lead the next day.
The question was now open as to what measures Major League Baseball would take. Already, however, measures taken by certain states and municipalities to ban large gatherings, including Washington and the city of Oakland, CA, meant that scheduled opening day series for the Seattle Mariners and Oakland Athletics would need to be moved to other sites. On March 12th, MLB announced that it was stopping spring training immediately and delaying Opening Day, scheduled for March 26th, by two weeks at least; the 2020 World Baseball Classic Qualifiers, which were supposed to get under way that same day were also postponed with no new date announced in the immediate. Minor League Baseball followed suit and announced that it was delaying the opening of its season as well (its first games had been scheduled to start on April 9th). The NCAA soon announced that the college baseball season would not resume and that the 2020 College World Series had been canceled.
On March 13th, the news broke that Major League Baseball, while still talking with the players' union, had suggested that players go to their homes, although that was not an order. Players could instead stay in spring training camp, or go to the city where their club plays home games. 
On March 15th, a New York Yankees minor leaguer became the first known coronavirus case in professional baseball.  A second Yankees minor leaguer tested positive a couple of days later . A number of employees of other teams were also revealed to have tested positive.
On March 16th, MLB Commissioner Rob Manfred announced that the start of the season would be delayed until mid-May, at the earliest. There was now talk of not holding the amateur draft this year, in order to reduce costs and because it would be impossible for teams to prepare properly for it. On March 24th, the pandemic claimed its biggest prize in the sporting world when it forced the postponement of the 2020 Olympics by a year. Until mid-March, the host Japanese government had been steadfast in ensuring everyone that the games would be held as planned in July, before also bowing to the inevitable due to concerns about safety. Athletes were unable to train while qualifying events were canceled around the world and a number of national federations indicated that they would not send athletes if the games went ahead, forcing the hosts' decision.
As of March 26th, which should have been Opening Day, there was still some hope that MLB would be able to play almost a full season, potentially utilizing lots of doubleheaders and perhaps playing all the way into the winter.  In one of the few positive notes, players who were injured had extra time to heal, and some who were expected to miss time could be ready when the season did start. 
- Anthony Castrovince: "MLB jerseys to be used to make masks and gowns", mlb.com, March 26, 2020. 
- Mark Feinsand: "Opening Day delayed at least 2 weeks; Spring Training games cancelled", mlb.com, March 12, 2020. 
- Gabe Lacques: "MLB is up next and must take immediate action in face of coronavirus crisis", USA Today, March 12, 2020. 
- Gabe Lacques: "MLB's journey into the unknown: Here's how baseball's coronavirus-delayed season could play out", USA Today, March 12, 2020. 
- Bob Nightengale: "'Not good for the game': MLB players express concerns about impact of coronavirus", USA Today, March 10, 2020. 
- Bob Nightengale: "MLB's coronavirus shutdown continues: Could season be pushed back several months?", USA Today, March 15, 2020. 
- Jeff Passan: "California order on gatherings of 250-plus could affect MLB games", ESPN.com, March 12, 2020. 
- Tom Schad: "2020 Tokyo Olympics officially postponed due to coronavirus outbreak", USA Today, March 24, 2020.