In this answer I'll start by explain what's the role of acceptance tests (AT) in the context of user stories give an example and finalise by answering your question.
To note this answer is adapted from the content provided by the Coursera course "Principles of Agile Software Development", from Instituto Tecnológico de Aeronáutica.
The acceptance tests are the result of the conversation between the Product Owner (PO) and the development team. In our case, they will look to see if the user story is satisfied (just looking at the result), not being interested how the product was coded (it is assumed / expected the code to have quality).
Another thing worth mentioning is that acceptance testing is implemented during Sprint. During Sprint, the development team has to develop the user story and make use of these ATs, so it's highly advised to have the acceptance tests before starting to code. Still, there may be some user stories that one can't set good acceptance tests at that time (for instances in Sprint zero). In certain situations, the development team may also suggest the PO new acceptance tests. If he/she accepts to be the owner of that test and say that it is valid, then it is valid, otherwise it is not.
Acceptance testing will help the PO and the development team know if what they developed for the user story is correct or not (that's what the acceptance test is for).
Generally speaking, the development team, in that sprint, already have the ATs and US they will implement. The first thing they do is implement the user story; then, they code the acceptance tests.
Example of AT
Imagine that you have a video streaming application to run on smartphone, tablet or computer. When you are watching a video streaming program and, for example, your mobile phone battery runs out, you would like to resume on a tablet, or another mobile phone, so without forcing you to enter the menu and go through many steps to get to the point you were.
We want to solve this problem, that is, when you switch from device to device when you are watching video streaming in this application that is going to be made, you start again from exactly the point you had stopped before, no matter why you had stopped before, you will start over from that point.
The user story can be as following
As a user of video streaming (role), I want to start watching program
on my mobile device, then I want to be able to resume the same program
on another mobile device at the point I stopped at the previous one
(goal), so that I don't miss a second of the show I was watching
I get off device, get on the other and already have access to the point where I had stopped, no matter why I stopped.
Some of the acceptance criteria to make sure that the US presented is working or not can be:
- If there was streaming video being streamed in the last hour and the user starts another session of the app, the system should ask the user 'Would you like to resume streaming from where you recently stopped?'
- If the streaming video he was watching is over (for instances, in the case of live events), the user will not see the resume message from previous stream, just the main menu of the app.
Scrum Team without testers?
In Scrum exists only three roles, Scrum Master, Product Owner and Development team. In your case, the person currently sick is part of the development team.
The development team is responsible to develop the user stories and implement the ATs (in certain cases also create ATs which then need to be approved by the PO).
So, anyone that's part of the development team can test if what was developed for the user story is correct. Once this point arrives, there's a ceremony called Sprint Review / Demo. In this ceremony, development team demonstrates to the PO and other interested parties what was implemented for the specific USs. The PO is the decision maker, and can then say "This US is done" or he/she is also free to change his/her mind and say "This US didn't go as i thought, the direction isn't the one i want and we need to do some changes" (considering not done).