...because people are selfish pricks.
As an example, oh say..take this guy, to protect his identity we'll call him Personal Injury Attorney Andrew S. On May 10th of this year he was told by the CDC he was infected with multi-drug resistant Tuberculosis, and told not to go on any flights as it would take a while to determine what kind of TB it was. Instead, he flew to Rome in Mid-May with his new wife for their honeymoon. The CDC then tracked him down in Rome and told him that his form of Tuberculosis was actually the extensively drug-resistant form (the most dangerous type), and to turn himself in to Italian health officials or US Embassy immediately for quarantine. Instead of doing that, he avoided Italian health officials and bypassed the USA's no-fly list (which he was on) by flying to Canada, getting on two separate flights, and then drove into the US.
Upon arriving in the United States, he then contacted the CDC about his condition. Of course, this is after he was already home, meaning that the people on those flights were already who-knows-where. Then the CDC flew him in a private plane to Denver for treatment (which they would've done from Rome), and now has been tracking down all the passengers from those two international flights and asking them to get TB tests (its hard enough to get people to call you back). Keep in mind a lot of these people aren't even US citizens, and the CDC is hardly swimming in cash.
Now, there's already plenty of reason to hate this guy...but, besides the fact that he's a lawyer, he also put all those other people at risk of contracting a highly dangerous infection, who by definition are traveling to all parts of the world. Its not like he walked into Lost Nation community center where the folks infected could be kept track of, he got on two separate flights of travelers who by definition were in transit to other places. Probably a lot of them to see their families. Probably quite a few hugged and kissed their kids when they got home, or will hug and kiss their kids sometime in the future when the bacteria has even more time to incubate.
Andrew's father-in-law works on highly drug-resistant TB at the CDC, coincidentally enough, but regardless of how Andrew got infected, he chose after being fully informed of its dangers, to put a lot of other people at needless risk of being infected. Now imagine this wasn't the fairly well-understood drug-resistant tuberculosis, but was the T-virus developed by the Umbrella Corporation. Now you're starting to see the big picture.
Even if a person is told in crystal clear terms that they have a very dangerous and infectious disease, they will still put many hundreds or thousands of others at risk if it means they can be slightly more comfortable. If even lawyers can't be trusted to care about the public good, what hope is there for us?