Any process in which the symbiont regulates its progression from an initial condition to a later condition, within the cells or tissues of the host organism. This may (but not necessarily) include a filamentous growth form, and also can include secretion of proteases and lipases to break down. The host is defined as the larger of the organisms involved in the symbiotic interaction.
Comment:
This term partially replaces the obsolete term 'regulation of growth or development of symbiont in host ; GO:0033665'. See also 'regulation of growth of symbiont in host ; GO:0044126'.