After your skin has been damaged, blisters are forming on the upper parts of your skin. You can see blister on every part of your body. They are pockets of fluid which are appearing on your damaged skin. But in the most cases these pockets are forming on your hands and feet. Blister can be filled with serum which is clear fluid. There are also blisters which are large than ½ inch. Those blisters are known as bullae. The period in which blister can last depends on the cause. Blisters which are caused by the irritations can go by their own within a few days and those blisters which are caused skin diseases and infections can last with weeks or months. But also these pockets can be filled with blood (they are known as blood blisters) or with pus if they are inflamed or infected. 
When to see a doctor? You should see your doctor if:
- Your blisters are very painful
- They have disappeared and they are coming back
- You are thinking that your blister is infected
If you have infected blister, then your blister will be red, painful and hot or it can be filled with green or yellow pus. If you have those symptoms, then you must visit your doctor. You should not leave the infected blister without treatment. If you are not having the right treatment for your condition, then your skin is having a risk for getting serious diseases such as sepsis and cellulites (in these cases people are infected with the contagious bacteria which is known as secondary impetigo and it appears in their skin). Also you should visit your doctor if you have noticed that your blister is not on the usual part of the body (hands and feet) such as inside of your mouth and on your eyelids and they are appearing when you are in contact with chemicals or you had burns, sunburns or allergic reactions.
What causes blister? This condition can be result of:
- Medical conditions such as impetigo  and chickenpox 
- Irritation of your skin
- Contact with chemicals such as detergent 
- Heat from a scald or sunburn
- Skin diseases
- Medications 
- Infections: There are many types of infections which can be reason for this condition such as
- Varicella zoster virus: This virus is a reason for the chickenpox. When this virus is a reason for the blisters, then they are starting with itchy rash and then this rash is developing in itchy blister. Also this virus can be cause for the herpes zoster or also known as shingles. When people are having herpes zoster, then they are having small and painful blister which is usually erupting in a linear pattern along the length of the infected nerves with varicella zoster virus. 
- Coxsackie virus: When people are having this type of infection, then they can have blisters on their mouth, hands and feet.
- Bullous impetigo: When people are having this type of infection, then their skin is red and the blisters are bursting easily. 
- Herpes simplex virus: When people are having herpes simplex type 1, then they are having cold sores or fever blisters which are appearing on the lips. Before blisters have appeared on the mouth, the skin can be red, itchy, swell and tingle. When those blisters are breaking, then they are leaking fluid which is developing in painful sores. When people are suffering from herpes simplex type 2, then this type of virus has been sexually transmitted. They are small red blisters which are appearing on the penis or in the vaginal area. 
- Allergies, irritation and burns: When blisters are caused by burns or friction, then they are painful. When blisters are caused by the eczema, then they are small bumps which are appearing on the infected skin and also they are red and can cause itching. 
- Skin diseases: Erythema multiforme is causing blisters on the mucous membranes in the mouth, eyes, genital and nose. Also this kind of blisters can appear in the forearms, the soles of the feet and on the palms of the hand.
- Medications: When people are exposed to sun and they are suffering from increased sensitivity to the sun, then there can appear blisters. In other reactions such as TEN blisters are involving the large areas of the tissue such as gastrointestinal tract and respiratory passages which are accompanied with malaise and fever. 
 National Institute of Arthritis and Musculoskeletal and Skin Diseases. Skin diseases. Retrieved from www.niams.nih.gov/health-topics/skin-diseases
 Pereira LB. Impetigo – review.Anais Brasileiros de Dermatologia. 2014;89(2):293–9.
 Centers for Disease Control and Prevention. About chickenpox. Retrieved from www.cdc.gov/chickenpox/about/index.html
 Uter W, Werfel T, White IR, Johansen JD. Contact allergy: A review of current problems from a clinical perspective. International Journal of Environmental Research and Public Health. 2018;15(6):1108.
 Shen Y, Nicoletti P, Floratos A, et al. Genome-wide association study of serious blistering skin rash caused by drugs. The Pharmacogenomics Journal. 2012;12:96-104.
 World Health Organization. Herpes simplex virus. 2020. Retrieved from www.who.int/news-room/fact-sheets/detail/herpes-simplex-virus