ansteckend
contagious
Refers to a disease that can be transmitted from one person or animal to another.
Die Grippe ist hochansteckend, deshalb solltest du zu Hause bleiben.
The flu is highly contagious, so you should stay at home.

contagious
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contagious
Refers to a disease that can be transmitted from one person or animal to another.
Die Grippe ist hochansteckend, deshalb solltest du zu Hause bleiben.
The flu is highly contagious, so you should stay at home.

The word comes from the verb 'anstecken' (to infect), which means 'infizieren' (to infect). 'Ansteckend' (contagious) is the present participle form used as an adjective and describes that something has the quality of 'anzustecken' (infecting/spreading to others).
Halte Abstand, ich bin ansteckend!
Keep your distance, I'm contagious!
You can tell whether the literal or figurative meaning is intended from the noun. If 'ansteckend' (contagious) refers to a disease or a virus, it is literal. If it refers to a feeling or behavior (laughter, yawning, good mood), it is meant figuratively.
Masern sind eine ansteckende Krankheit.
Measles is a contagious disease.
To emphasize that a disease is very easily transmissible, the adverb 'hoch' (highly) is often placed before it. The combination 'hochansteckend' (highly contagious) is very common.
Windpocken sind eine hochansteckende Kinderkrankheit.
Chickenpox is a highly contagious childhood disease.
In the figurative meaning, 'ansteckend' (contagious) is often combined with positive emotions and spontaneous reactions. Typical phrases are 'ansteckendes Lachen' (contagious laughter), 'ansteckende Freude' (contagious joy), 'ansteckende Begeisterung' (contagious enthusiasm), or also 'ansteckendes Gähnen' (contagious yawning).
Sein Enthusiasmus für das Projekt war total ansteckend.
His enthusiasm for the project was totally contagious.
transferable / transmissible
'ansteckend' and 'übertragbar' can be very similar when talking about diseases. Both say that a disease can be passed from one living being to another. 'ansteckend' is very natural in everyday language, especially when talking about sick people. 'übertragbar' sounds somewhat more factual or medical. In addition, 'übertragbar' is broader: it can also be used for rights, tickets, or ideas. That is why the words can be confused. With diseases, both are often possible, but in everyday speech people usually say 'ansteckend'.

Die Ärztin sagt, dass die Krankheit noch einige Tage ansteckend ist.
The doctor says that the disease will still be contagious for a few days.
Diese Infektion ist durch engen Kontakt übertragbar.
This infection is transmissible through close contact.