"There is no return. This. Is. Terminus.”
I will never fully understand how the heck Tegan managed to get away uninfected here, really.
I always assumed her symptoms in Enlightenment were a lighter form of Lazar’s (thank god for the TARDIS’ medbay?), and Nyssa only got so bad so quickly because Trakenite immune systems are just. Just really bad. Really, really bad. Oh, Nyssa.
Different genetic markers allow different people to resist disease in different ways. Sickle Cell Anemia, for example, makes people who have it strongly resistant to malaria. Also, it’s possible that Lazar’s is not very contagious and Nyssa was just damned unlucky. This episode aired during the first horrific days when AIDS was coming to the public’s attention, and there was SO MUCH FEAR because it was usually a death sentence back then, and people were afraid to touch people who had HIV, and schools would expel kids who had HIV because they were afraid the kid would give it to everyone, and ditto workplaces — nobody seemed to understand that just because the disease was (then) lethal if you caught it, didn’t mean it was easy to catch it.
Maybe Nyssa had a papercut on a finger, or the chemicals she was using in her experiment made it easier for the stuff to get through her pores (like a nitro patch), or she just happened to be that statistic of the one in ten thousand people who contracts it by breathing it.
Big Finish provides a further reason. At one point, Nyssa’s son takes Tegan aside to collect a blood sample (with her permission), because she has immunities to pathogens that were stamped out hundreds of years ago, and because she has no synthetic DNA so her biology is nothing like anyone in his time period. To them, she’s like someone who lived in medieval Europe and was exposed to various plagues and unsanitary conditions that would probably wipe them out because their own immune systems can’t cope.
Whereas Nyssa tends to be particularly susceptible to this and that because she lived on Traken which pretty well had stamped out disease and had very little exposure to offworlders. (In fact, if she hadn’t run into the Dar Traders and had weird things done to her own natural immunities, or lack thereof, she WOULD have died.)