If I am contagious and I touch my face or my mask I will spread germs. If I wear a mask on my face it will collect some germs (some will still escape) and make it easier for me to get them on my hands and spread them. At the end of the day, if I am contagious and I breath, I will spread germs. Your best defense is to boost your immune sytem so that you too will be asymptomatic.