While absence due to communicable illness is not only acceptable but encouraged by educators, I feel most in our profession would agree that students should not be allowed to miss more than a previously-specified number of days without cause. While an occassional absence for a personal day is understandable, frequent student absences not only result in poor academic achievement for the student, but the combined frequent absences of several students within the same class cause significant difficulties for the educator when it comes to the effective design, implementation, and assessment of his or her instruction.
Effective instruction is based on the collection and analysis of data: what do the majority of students know and not know (prior knowledge); what do they need to learn (curriculum standards) and how (best practices / effective instructional approaches); how will teacher modeling, guided practice and independent practice be designed; and what is the result of student assessment. When several students have several absences within a unit of study, the data the teacher is collecting becomes skewed, often resulting in a false negative showing that the teacher will need to reteach a given concept (or concepts) to the entire group, thus holding back the students whose attendance has not been a concern.
Conversely, if the group of students with frequent attendance concerns are not present when assessment data is collected, the class performance could result in a false positive, making it appear as though the teacher has thoroughly instructed the students to mastery. However, the absent students would most likely have missed several key concepts within a unit, and therefore not have mastered the material.