How contagious is strep throat?

Yes: Yes it is. Generally thru close contact through aerosol spread from breathing, sneezing, etc. Also perhaps through contact with a glass, toothbrush and the like. Also from material on the infected persons hands.

Family attack rate: Family transmission rates have been stated as low as 1/32 (summer) to as high as 1/4 (winter) for transmission from symptomatic patients per 24 hr of pre-treatment exposure. Some can actually carry it without becoming ill.Transmission is primarily from contaminated secretions compared to chickenpox that passes easily in air born droplets.