Masks protect against respiratory transmissible biological agents. The mechanosm is not avoiding any possible contagion, but limiting (i.e. reducing) contagion in a way that depends on the infectious dose. That is, they reduce thebdose of microorganisms emitted or ninhalednl by each person in the community. Reduced infectious doses are better sealt with by the innate immune system. Thus infection antlow doses may not develop into clinically manifest disease. Thus,  also subsequent transmission is reduced. During an epidemic, masks must be weared continuously whe in presence of several persons within closed spaces. In open air, they must be weared when many persons are close to each other (small events, etc.). Large events shouldnbe avoided.

B. If they are or may be infected. Masks will prevent or reduce droplets emitted while talking, singing, or sneezing. With masks donned the risk of airborne or droplet transmission is significantly reduced. Several factors would interfere with infection transmission, ventilation, humidity, and temperature affect how long and how far droplets are suspended in the air. Also, the immunity status of individuals and the public would indicate the level of risk of infection transmission for the respiratory virus.
A. Masks are a protection: Masks are categorized among personal protective equipment. They are at the bottom of the hierarchy of the inverted occupational hazard control pyramid. They protect the person using it and the one in close contact. 

If you are sick, stay home, don't go to work. If traveling and sick with respiratory symptoms, wear a mask. I always wear a mask when flying since I don't know the person (s) next to me.  Masks are for protection (yourself and others), not a social signal.

JP KABUE

1.c
2.a

I am a veterinarian. Work with fowls that have Avian influenza, pigs with Porcine Influenza. Also check out huge numbers of animals/ birds in production houses. ALWAYS wear a mask AND full protective clothing TO PROTECT MY PATIENTS as well as preventing me from being a carrier. In September 2023 I had a routine blood test. The paramedic was coughing and did not wear a mask. Two days later I was positive for  COVID-19 Omicron  strain.  I WAS VERY ANGRY with the paramedic!!! As a patient , I will now ALWAYS wear a mask if I visit a medical practice in future. Its obviously full of sick patients and paramedics with no face masks. SELF DEFENCE.

Given that people can transmit respiratory viruses even when they are experiencing zero symptoms, masks should always be worn when in public. It's an enormous failure of public health that mask mandates have been dropped. Masks work best when everyone wears them, and they need to be high quality masks (FFP2s, N95s, KN95s, etc.) Baggy blues don't offer much protection, but properly worn high quality masks DO.

When most people are unaware who is infected or not, one needs to put on a mask as that shall surely reduce the nasal uptake of contagious particles. It holds true and right for the respiratory viruses.

Wearing of mask is needed for self protect as well as for others protection.

a,a both protection and help others if you have infection

A) Always. 

2.Masks are  a) protection

For Respiratory Virus stablish Folks always should wear masks

1. b & c
2. a

If afraid then must put on mask always, because in that case your immune system will be further weakened by the fear of being catching infections.

B) If they are or maybe infected, so that they do not transmit it ; A) Protection : Masks are usually a protection

only in healthcare setting or other places where respiratory protection programs can be operated, and everyone is trained and fit-tested. otherwise, it is just a waste of effort, unless new evidence emerges showing that this is not so. mass masking during covid-19 (and prior similar epidemics) seem to only have cause distress (physical and mental), societal strife, and sense of false security.  all this was written about many times many years ago by infection control community: they only way to reduce risk from such viruses is a safe and effective vaccine.

1: B
2: A

b)