PEO's (Pure Essential Oils) are very powerful compressed oils from botanical sources that can be used with good effects with proper administration & a little experimentation of what works for you. You specifically mention immunity, Using PEO's with antibacterial properties is a place to start. Two well known PEO's with these properties are Tea Tree Oil & Mint. (Less bacteria=Less vulnerability to a compromised immunity.) Lavender is also antibacterial, as well as calming, soothing properties. A few things to be aware of please: 1) Get a good book to learn how/which PEO's to mix (it is measured in drops & certain oils are compatible & non-compatible); 2) Make sure (especially if you buy in a health food store) that you are getting a PEO. Many places sell EO's mixed in a base oil like jojoba or grapeseed which is fine when applying directly to the skin-but be warned you are paying for the base mostly with very little EO, therefore, very little therapeutic value in them. Better to get PEO (keyword "Pure") & mix it yourself if needed. Many sales people in stores are unaware of the difference or just ignore the fact. 3) More does not necessarily mean better but can mean you will have a skin reaction. Some things my family uses: Always have Lavender, Peppermint & Tea Tree Oil in the house. Most economical to buy the larger bottles. (Keep out of sunlight & cooler places to store not refrigerate.) These 3 can be used in many ways to help. Examples: Suffer from migraines? As soon as you realize one is coming on, grab the mint & put on tissue/hankie or just open up that bottle & inhale. This can sometimes avoid an episode. Feeling stressed out like you're not quite yourself & might be coming down with something? Take a warm bath adding 2-3 drops peppermint, 5 drops lavender, 3 drops chamomile; no chamomile-use 2-3 drops peppermint, 8 drops lavender. Wash with a lavender soap or a soap with no scent. Inhale deeply. Any fungal problems like athlete's foot or skin rashes, acne? Then Tea Tree Oil is your new best friend. Also in baths with the flu/colds, etc. This is an antibacterial like mint so use with caution & balance with a soothing oil. Look up Shirley Price Aromatherapy online. Good info, good company, good products. Good Health!