Plague Inc. List of Symptom Combos and Respective Guides:

 Symptom Combos are when you get certain symptoms (and in some cases, transmissions) that affect your disease in some way. This could be an increase/decrease in infectivity/severity/lethality or make your disease easier/harder to cure. Some symptoms combos can only happen in a certain diseases. I am fairly sure that if you want a certain symptom combo, just having the respective symptoms will make it easier. However, you should never avoid evolving other symptoms/abilities/transmissions to get a symptom combo. Effects of symptoms can not be reversed.



Symptom Combos in Regular Diseases:

The Projectile Vomiting Combo is obtained when coughing and vomiting are evolved. This increases the infectivity of your disease slightly. A popup that say
"Coughing and vomiting are causing the infected to projectile vomit, increasing the infectivity of [disease name]" will appear if you have the combo. This unlocks the Long Shot achievement.
Projectile Vomiting

The Swine Flu Combo is obtained when Coughing, Diarrhoea, Insomnia, Nausea, Pneunomia, Vomiting and Livestock 1/2 are evolved. This slows down the cure slightly. A popup that says:
"[Disease name] has similar symptoms to Swine Flu. Doctors researching the cure are wasting time looking into cures for Swine Flu. Cure will take longer" will appear if you have the combo. This unlocks the Oink Oink achievement.
Swine Flu

The Oops Symptom Combo is obtained when Diarrhoea and Sneezing are evolved. This increases infectivity and Severity and slows down the cure. A popup that says
"Sneezing and diarrhoea are causing unwanted accidents, lowering productivity and drawing attention to [disease name]" will appear if you have the combo. This unlocks the Uh Oh achievement.
Oops

The Profuse Bleeding Symptom Combo is obtained when Haemophilia and Skin Lesions are evolved. This increases lethality and infectivity. I am fairly sure infectivity gained is increased more if blood 1/2 is evolved. A popup that says
"Haemophilia and skin lesions are causing the infected to have blood pouring out of their skin, making [disease name] far more dangerous" will appear if you have the combo. This unlocks the Red Rain achievement.
Profuse Bleeding

The Walking Dead Symptom Combo is obtained when Anaemia and Insomnia are evolved. This slows down the cure slightly. A popup that says
"Insomnia and anaemia are causing people to walk around grey with tiredness. Scientists working on the cure for [disease name] are less productive" will appear if you have the combo. This unlocks the Brainzzzz chievement. This combo is not related to the Necroa Virus.
Walking Dead
The Public Defecation Combo is obtained when Insanity and Diarrhoea are evolved. This increases infectivity but speeds up cure research.  A popup that says "Diarrhoea and insanity are causing the people to defecate in public (and everywhere else!) Everyone is increasingly focused on developing a cure" will appear if you have the combo. This unlocks the Brown Streets achievement.
There is no guide yet for Public Defecation.

There are no Neurax Worm unique combos.

Necroa Virus Combos:

The Vampire Bat Combo is obtained when Vampric Hunger and Bat 1/2 are evolved. This increases Severity. This unlocks the Is it a bird? achievement.
There is no guide yet for Vampire Bat.

The Walking Contradiction combo is obtained when Cathemeral Shift and Photophobia are evolved. This decreases severity. This unlocks the Walking Contradiction achievement.
There is no guide yet for Walking Contradiction.

The Blood in the Air combo is obtained when Delirium and Hyperosmia are evolved. This increases severity. This unlocks the Jaws achievement.
There is no guide yet for Blood in the Air.

The Cranial Dispersion combo is obtained when Air 3 and Cranial Elephantitis are evolved. This increases severity. This unlocks the Use your Head achievement.
There is no guide yet for Cranial Dispersion.

The Boomer combo is obtained when Liquefactive Necrosis and Autothysis are evolved. This increases severity. This unlocks the Boomer achievement.
There is no guide yet for Boomer.

The Tank combo is obtained when Anabolic Boost and Bone Dysplasia are evolved. This increases severity. This unlocks the Tank achievement.
There is no guide yet for Tank.

The Runner combo is obtained when Engorged Quadriceps and Enhanced Motor Control are evolved. This increases severity. This unlocks the Runner achievement.
There is no guide yet for Runner.

The Spitter combo is obtained when Enhanced Sensory Integration and Naja Mortis are evolved. This increases severity. This unlocks the Spitter achievement.
There is no guide yet for Spitter. 

The Bath Time combo is obtained when Polyphagia, Autophagia and Cannibalism is evolved. This increases infectivity. This unlocks the Tasty achievement.
There is no guide yet for Bath Time.