When is cold and flu most contagious ?

It depends . Some people are more contagious before they show symptoms of a cold or flu. However that may not be the time when they are most contagious. You may be contagious before you know you are sick. It all depends on the virus. In some cases you will be  aware of your illness before you pass it on. In some cases you are contagious as long as you have the symptoms or even beyond. With some others , you continue to have the symptoms after you are  no longer contagious.

Adults who have  the flu are contagious for five days after first developing the symptoms. Kids can stay contagious even longer. If you have a cold, you are most likely to give it to someone else  during the first few days of runny nose , sneezing and coughing.

It is important to practice good hygiene such as washing hands frequently  even when you seem perfectly healthy.