How likely is it that companion animals will become infected with one of the viruses causing chronic diseases like cancer in humans?

There is a less serious problem in companion animals compared to humans because they are not directly targeted by the Extraterrestrial Alliance, but there can still be exposure to the same viruses that affect humans at times as well as other triggers for malignancy taking place. So this becomes a matter of personal choice in how thorough one chooses to be in helping to protect their pets. Every situation is unique in some respects with regard to the level of exposure and the threat it represents, what the response of the animal might be in handling a chronic viral infection, or even production of a malignant tumor. So we cannot make a general recommendation other than saying that the recommended Antiviral Regimen is safe to use in pets if adjusted on a body weight basis to use doses proportional to their smaller size, and would be advantageous if affordable to use chronically, just as with people.