What does a welder protect against when he bears a mask?

From what does ozone layer protect earth?

A number of infectious diseases have ill-effects on human health while some of these are fatal. AIDS is one of the highly serious pandemic diseases. Though a number of drugs are available but none of the these provides absolute protection against the disease. Discuss the following: What are dificulties against developing the AIDS vaccine?

When does a drought occur?

When does a bulb glow ?

These play role in protecting body against diseases.

What are antibiotics? How they act against pathogens (bacteria)?