Consuming left over onion is poisonous

Research has shown that, sliced left over  onions seem to always contact bacteria during the period it is unused. There by leaving the onion poisonous. Consuming such onion tends to expose us to viral diseases with time.

Is it therefore advisable not to leave already sliced onions if we don't intend to use it ASAP.

It is not even safe to try and keep it in a refrigerator or closed bag.. Finish using the onion as soon as you cut it open