What if an opossum is seen during the day does it mean that its sick?

Typically no, since a more likely reason for an opossum to be out during the day is that it was disturbed where it was sleeping or is trying to find a better place to stay, however it is ill advised to approach an opossum during the day in the event they are sick. It's uncommon for opossums to get rabies, but being out in daylight is a potential indicator of the disease.