Thank you for your question. You can catch Herpes by coming in contact with the blisters or body fluids of someone who is actively shedding herpes virus. This is usually the 7 days prior and three weeks after they get blisters. You can also catch it from someone with a chronic infection but no blisters. There are two types. The labialbone is often caught by children when adults kiss them. The virus can either cause blisters or stay dormant till a fever appears. Genital herpes can be caught when having sex with someone having the virus. This too can result in blisters or stay dormant till there is a change in immunity e.g medication or pregnancy or stress or malnutrition etc A blood test can determine of you have an active infection or an old one but not when you got it.