Before I even get on to ptsd. Everyday people who have had lovely lives and never had a hint of mh issues before are struggling just now because this is a stressful and hard thing we are all living through. For everyone else this is ontop of what we already have to deal with!
PTSD causes uncontrollable reactions to certain things as we experience different things in life sometimes some of those things become "triggers" we don't necessarily know why these seemingly unrelated events or experiences cause a trigger reaction but the likelyhood is they somehow cause a similar emotional reaction as something that originally caused the ptsd.
So a global pandemic is an unsurprising trigger for the symptoms of ptsd. The emotional responses would be very similar to the heightened feelings that your partner has already experienced having been forced to live through during combat. Partly because both are long term events (with not much an individual can control) so there is no chance to get away from the feelings so they build and build.
The other possible reason could be that It has for lots of people caused them to feel like they are living in a "war zone" the government has even compared this even to being at war.
I am not saying the two things are the same but the language used could theoretically cause the brain to link the two events as similar which would then cause both to trigger the ptsd response.
Your partner needs to get some help, there is nothing to be ashamed about. The illness has linked the two unrelated events. There is no reason in the world why your partner shouldn't think it's a perfectly normal thing to go and get seen about.
have you done any reading on ptsd so you have the language to be able to talk to you partner? its not your job but if you had the knowledge you may be I. A better position to steer your Dp towards getting help.