Again, just because you don't see the disease, doesn't mean it's not there. How you're feeling may not be visible, but that doesn't mean you don't feel extremely tired, that you don't end up exhausted by climbing a few stairs, or that you don't feel sick to your stomach. Even a fever can be "invisible." It's totally normal to suffer from symptoms that stop you from doing what other people do, even though they may not show. Discover 10 tips to help you through cold and flu season.