To take the example of Japan according to Gapminder 1918 had a huge drop. The other big drop is the is the second world war with all the bomb dropping and shooting that involved.interesting point here about how the internet can help prevent and reduce epidemics.
"The internet can be our key to survival because the ability to work telepresently can inhibit microbial transmission by reducing human-to-human contact. In the face of an otherwise devastating epidemic, businesses can keep supply chains running with the maximum number of employees working from home. This can reduce host density below the tipping point required for an epidemic. If we are well prepared when an epidemic arrives, we can fluidly shift into a self-quarantined society in which microbes fail due to host scarcity."
Eagleman has a good short video on his thesis
The long term effects of a sudden switch to everyone avoiding each other for a month or two could be huge. These would include
Education: How schools help spread influenza has been studied. 'School closures during the 2009 influenza pandemic: national and local experiences'. If all the schools were closed for a few months and people would move to Khan Academy and other online education sites. After this period a switch back to a fully non online world won't happen
Telecommuting: In a similar way online telecommuting would become much more popular. After a quarantine lite period the use of online project management and other telecommuting tools would become mainstream.
Shopping: If you don't meet people in school or at work you meet them in the shops. Deliveries of shopping would be strongly encouraged in the event of a pandemic. They should probably even be sponsored. Shops would not get as popular again once everyone got used to online shopping.
Banking: No one likes queuing in the banks at the best of times. Even ATMs would become horrible grubby in a pandemic world. Everything including social welfare payments would try and avoid using the fomite that is cash.
Telemedecine: People with the influenza need to be kept away from people who are sick. People with other illnesses will have to be dealt with remotely to avoid them coming into contact with people with influenza.
Public Events: Public events parades, cinemas, bars and museums would be closed. By their nature these involve people. If public events are made cheaper to attend virtually that will reduce the need for people to meet up. By this I mean if Sky Sports is made free for a few months people will be less annoyed no fans are allowed attend the football game.
There are many people without access to the internet that would not be helped by the use of digital technologies. Hopefully the use of digital technologies will help focus more of the traditional public health effort on them.
When the next pandemic happens the internet will reduce the consequences. Many industries will also change but the main thing is to avoid the 50 to 100 million the last pandemic killed.