Is it obligatory to perform tayammum for prayer when afflicted with chickenpox, which prevents the use of water, given the doubt about the effect of water on the spread of the pustules?

Tayammum is permissible if using water would cause harm, such as worsening an illness or delaying recovery. This can be known through experience or by the report of a trustworthy doctor. It is recommended to consult more than one doctor to confirm the effect of water on the illness. If a specialized doctor determines that using water would be harmful, then there is no objection to following their advice. However, it is preferable to consult another doctor as a precaution and for confirmation.