Asymptomatic transmission is common in colds, flus, and stomach viruses. So this shouldn’t surprise anyone. Also, up to 5 days after colds and stomach bugs you can still transmit the germs.
Most sites say 1 day prior to symptoms for the flu. And it usually stops being contagious as the symptoms go away. There are exceptions. Kids tend to spread it longer afterwards. What they saw in Germany was worse than the typical flue or cold pattern.
"When Flu Spreads. People with flu are most contagious in the first three to four days after their illness begins. Most healthy adults may be able to infect others beginning 1 day before symptoms develop and up to 5 to 7 days after becoming sick." - CDC
"For colds, most individuals become contagious about a day before cold symptoms develop and remain contagious for about five to seven days. Some children may pass the flu viruses for longer than seven days (occasionally for two weeks)." - Medicinenet