Complex sentence: Smallpox was eradicated by a massive international search for outbreaks, backed up with a vaccination program which started in 1967, organised and co-ordinated by a World Health Organisation (WHO) unit set up and headed by Donald Henderson.
Split into simpler sentences: Smallpox was eradicated by a massive international search for outbreaks, backed up with a vaccination program, starting in 1967. It was organised and co-ordinated by a World Health Organisation (WHO) unit, set up and headed by Donald Henderson.