We adopted Tiddles nearly 3 weeks ago and he started sneezing, but he also started to get discharge from his nose. After being tested we have found out he has 3 bacteria infections, one of which is E-coli and is now on antibiotics - as are our other 3 cats as they started showing symptoms too.
I seriously recommend that you get them checked out by the vets as it could be anything. Luckily Tiddles (now nearly 12 weeks old) is on the road to recovery as we caught it quick - but he's not out of the woods yet.
I hope they all get better soon, but it could anything so it's better to be safe than sorry and go to the vets. xxx