Read Marla Apt's response:
As yoga teachers, we have to first tend to ourselves before we can be of service to our students, which is why it is good that you took the necessary time off while sick. But what you teach in class should be tailored to your students. Therefore, regardless of your physical abilities or energy level, you are teaching to the level of the students in front of you. Even if, due to injury, illness, or other limitations, the students are more able-bodied than the teacher, the knowledge of the teacher (not just from study but from practice) should always exceed the student's knowledge. This is why they've come to learn from you. You don't need to do the practice along with the class. In fact, most of the time you'll be much more effective if you watch and help your students rather than practice alongside them. If they need to see a model and you're not strong enough to demonstrate, you can ask one of the more seasoned students (or an assistant, if you have one) to stand in front of the class and mirror the students—leaving you free to watch the class from optimal viewing angles and help as needed.