If you are using Scrum/Agile you are working as a team. The sprint is a team effort where you work together towards a common goal and deadline. You work of the list of items in you backlog based on priority/importance one by one until you are done.
With a cross-functional and self-organizing team there is usually no specializations so everyone can pick up any task. So if someone would become sick everyone helps out, and try to do the best they can to meet the common goal/deadline. That someone is sick becomes obvious on the daily standup meeting if not before.
With that said, if someone become sick the velocity for the given sprint will be affected, i.e. you can't handle as many tasks. You need to figure out what the impact will be in your sprint, and how this can be handled as a team.
To reflect the absence in VisionFlow you should start by updating the resource availability for the current week (Project --> Sprint --> Manage resource availability), such as reducing the number of work hours from 40 to a more accurate number. This will make it visible on the sprint that you probably will not be able to complete as many tasks as you initially tought, i.e. you can compare the resource availability (hours) with the estimated hours for the sprint.
Normally you keep the sprint backlog intact and don't remove items from the backlog to reflect resource availability, it will then become clear at the end of the sprint what you could not complete and why. The items that could'n be completed in the sprint will then be moved over to the next sprint.