Different variations of the statement: “Face masks are bad for your immune system. Bacteria multiply in face masks. Face masks cause diseases. The WHO did not initially recommend the use of face masks. "
There is currently no evidence that wearing face masks weakens the immune system in any way. It is also nonsense that CO2 would accumulate under the face mask and harm us. The CO2 molecules are so small that it is no problem for them to go through the mask. Only bacteria that you exhale yourself will stay inside the mask. If you use it properly and wash or boil it after each use, there can be no significant multiplication of bacteria.
Nor is there any evidence that the masks can cause any diseases. The only possible health problem caused by wearing a mask may be an allergic reaction of the skin to the material from which the face mask is made (if you suffer from such an allergy).
Yes, the WHO did not recommend the use of face masks in the first wave. But it was not because it was ineffective, but for fear of misuse. When used and worn properly, face masks are an effective protection against the sprea of coronavirus according to the WHO.
Here is an explanation of the whole hoax (in Czech)