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Ageing and age-related diseases (ARD) share a common biological clock that appears as their root cause: protein damage. A majority of proteins have evolved into native structures resistant to oxidative damage but any folding imperfections, including those due to “silent” amino acid substitutions, reduce oxidation resistance. Damaged proteins accumulate with age and trigger ageing-like phenotypes reversible by their turnover, while acquired genome alterations remain as stable consequences of protein malfunction. Ageing and ARD display species-specific latency in phenotypic expression. Disease latency may be proposed as to be due to phenotypic suppression of cellular defects by molecular traffic among neighbouring cells. Such cross-complementation of functional deficiencies acts as a kind of tissue-based cellular “solidarity”, called cellular parabiosis. Chronic inflammation reveals dormant cell phenotypes and shortens disease latency by the breakdown of cell-cell communication, as in tumour promotion and inflammation. At the present time, predictive diagnostics, prognostics, prevention and even cure of disease by phenotypic reversion become conceivable.