There has been global cancers before. With the difference that we may be able to restrain ourselves. As an example see the great oxygenation event. Where bunch

There has been global cancers before. With the difference that we may be able to restrain ourselves. As an example see the great oxygenation event. Where bunch of selfish green algea pumped waste product into the atmosphere killing almost all anaerobic life. Then there is the carboniferous period where trees grew so much that due to lack of lignin breaking fungi they basically sucked out all CO2 from atmosphere causing global glaciation and killing themselves in the process. Life has been crapping itself since the very start. We are no worse than others, however we have the chance to be better.