Читать книгу 100 Ways to Boost Your Immune System онлайн
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Even smaller than bacteria, viruses can only be seen through electron microscopes, high-powered instruments that are used to study very small objects. Unlike bacteria, viruses are not self-sufficient; they need a suitable host in which to reproduce. When a virus such as influenza enters you it takes over healthy cells and spreads through your body, causing illness.
Fungi and parasites
Slightly larger than bacteria, fungi live in the air, water, soil and on plants. Moulds and yeasts are types of fungi that can live in your body but don’t always cause illness. Some fungi such as penicillin – an antibiotic which kills harmful bacteria – have health benefits, but other fungi are not so good for you and can cause illness; an example might be Candida, which can cause yeast infections and oral thrush.
Protozoa are single-celled organisms that spend part of their life cycle outside humans living in food, soil, water or insects, and part of their life cycle living within your body as a parasite. Many protozoa inhabit the intestinal tract and are harmless, but others can cause digestive problems and disease.